Rajasthan Board RBSE Class 9 Science Important Questions Chapter 3 Atoms and Molecules Important Questions and Answers.
RBSE Class 9 Science Chapter 3 Important Questions Atoms and Molecules
Objective Type Questions
Question 1
Which of the following correctly represents 360 g of water?
(i) 2 moles of H2O
(ii) 20 moles of water
(iii) 6.022 × 1023 molecules of water
(iv) 1.2044 × 1025 molecules of water
(a) (i)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Answer:
(d) (ii) and (iv)
Question 2.
Which of the following statements is not true about an atom?
(a) Atoms are not able to exist independently.
(b) Atoms are the basic units from which molecules and ions are formed.
(c) Atoms are always neutral in nature.
(d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch.
Answer:
(a) Atoms are not able to exist independently.
Question 3.
The chemical symbol for nitrogen gas is :
(a) Ni
(b) N2
(c) N+
(d) N
Answer:
(b) N2
Question 4.
The chemical symbol for sodium is :
(a) So
(b) Sd
(c) NA
(d) Na
Answer:
(d) Na
Question 5.
Which of the following would weigh the highest?
(a) 0.2 mole of sucrose (C12H22O11)
(b) 2 moles of CO2
(c) 2 moles of CaCO3
(d) 10 moles of H2O
Answer:
(c) 2 moles of CaCO3
Question 6.
Which of the following has maximum number of atoms?
(a) 18 g of H2O
(b) 18 g of O2
(c) 18 g of CO2
(d) 18 g of CH4
Answer:
(d) 18 g of CH4
Question 7.
Which of the following contains maximum number of molecules?
(a) 1 g CO2
(b) 1 g N2
(c) 1 g H2
(d) 1 g CH4
Answer:
(c) 1 g H2
Question 8.
Mass of one atom of oxygen is :
(a) 16/6.023 × 1023 g
(b) 32/6.023 × 1023 g
(c) 1/6.023 × 1023 g
(d) 8 u
Answer:
(a) 16/6.023 × 1023 g
Question 9.
3.42 g of sucrose are dissolved in 18 g of water in a beaker. The number of oxygen atoms in the solution are:
(a) 6.68 × 1023
(b) 6.09 × 1022
(c) 6.022 × 1023
(d) 6.022 × 1021
Answer:
(a) 6.68 × 1023
Question 10.
The formula for quicklime is:
(a) CaCl2
(b) CaCO3
(c) Ca(OH)2
(d) CaO
Answer:
(d) CaO
Question 11.
Which of the following represents a correct chemical formula?
(a) CaCl
(b) BiPO4
(c) NaSO4
(d) NaS
Answer:
(b) BiPO4
Question 12.
Fill in the blanks :
(a) In a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is called …………….
(b) A group of atoms carrying a fixed charge on them is called ……………..
(c) The formula unit mass of Ca3(PO4)2 is ……………..
(d) Formula of sodium carbonate is …………….. and that of ammonium sulphate is …………..
Answer:
(a) Law of conservation of mass
(b) Polyatomic ion
(c) (3 × atomic mass of Ca) + (2 × atomic mass of phosphorus) + (8 × atomic mass of oxygen) = 310
(d) Na2CO3; (NH4)2SO4
Question 13.
Match the items in column A with column B :
Column A | Column B |
(i) Ozone | (a) K2S |
(ii) Potassium Sulphide | (b) 34 |
(iii) Al2(CO3)3 | (c) Triatomic |
(iv) Molecular mass of H2S | (d) Aluminium carbonate |
Answer:
Column A | Column B |
(i) Ozone | (c) Triatomic |
(ii) Potassium Sulphide | (a) K2S |
(iii) Al2(CO3)3 | (d) Aluminium carbonate |
(iv) Molecular mass of H2S | (b) 34 |
Question 14.
Match the items in column A with column B :
Column A | Column B |
(i) Atomic radius of Haemoglobin | (a) 2 mol oxygen |
(ii) Valency of sulphur | (b) 3.011 × 1023 sodium molecules |
(iii) 23 gm sodium | (c) 10-8 metre |
(iv) 32 gm oxygen | (d) 2 |
Answer:
Column A | Column B |
(i) Atomic radius of Haemoglobin | (c) 10-8 metre |
(ii) Valency of sulphur | (d) 2 molecules |
(iii) 23 gm sodium | (b) 3.011 x 1023 sodium |
(iv) 32 gm oxygen | (a) 2 mol oxygen |
Question 15.
Read the statements carefully and identify whether they are True or False-
1. Copper is a diatomic element.
2. The symbol of nitride ion is N3-
3. Dalton’s atomic theory was based on the law of chemical combination.
4. Atomic radius of hydrogen is 1 mm.
5. The molecular formula for potassium nitrate is KNO3.
Answer:
1. False
2. True
3. True
4. False
5. True
Very Short Answer Type Questions
Question 1.
State the law of conservation of mass.
Answer:
This law states that the mass can neither be created nor destroyed in a chemical reaction. That is mass of reactants is always equal to mass of products.
Question 2.
Who was the first scientist to give the concept of formation of compounds?
Answer:
Antoine L. Lavoisier gave the concept of formation of compounds.
Question 3.
Who stated the law of constant proportion?
Answer:
Joseph Louis Proust stated the law of constant proportion.
Question 4.
What is the measuring unit of atomic radius?
Answer:
Nanometre (nm) is the measuring unit of atomic radius.
Question 5.
Give the derivation source of symbol of sodium (Na).
Answer:
The symbol of‘Na’ for sodium is derived from its Latin name ‘Natrium’.
Question 6.
What is the latest short form of atomic mass unit?
Answer:
The latest short form of atomic mass unit is u, according to IUPAC.
Question 7.
Name any two monoatomic atoms.
Answer:
Sodium, Aluminium.
Question 8.
What do we get if 3 atoms of oxygen unite into a molecule, instead of usual 2?
Answer:
We get (O3) ozone.
Question 9.
Give difference between 2H and H2.
Answer:
2H indicates 2 atoms of hydrogen and H2 indicates one molecule of hydrogen.
Question 10.
Give two examples of triatomic molecules.
Answer:
Carbon dioxide (CO2) and water (H2O).
Question 11.
Define valency.
Answer:
The combining power of an element to attain the noble gas configuration is called valency. Or, it is defined as number of electrons lost or gained by an atom to acquire noble gas configuration.
Question 12.
What is Avogardo Constant?
Answer:
The number of particles present in one mole of any substance is fixed with a value of 6.022 × 1023.
Question 13.
What is ion?
Answer:
An ion is a charged particle. It can be positive or negative.
Question 14.
What do you mean by symbols of elements?
Answer:
Each element is represented by a letter or group of two letters to write the chemical reactions conveniently. It is called symbol.
Question 15.
Give the symbol of copper, silver, gold, oxygen, zinc.
Answer:
Copper – Cu,
Silver – Ag,
Gold – Au,
Oxygen – O,
Zinc – Zn.
Question 16.
Name two elements which have same atomic number.
Answer:
Two elements cannot have the same atomic number.
Question 17.
Write the value of charge of electron.
Answer:
1.6 × 10-19 coulomb.
Question 18.
Name the gas which gives the lightest positively charged particle.
Answer:
Hydrogen.
Question 19.
Define law of conservation of mass.
Answer:
Law of conservation of mass states that, ‘Mass is neither created nor destroyed in a chemical reaction.’
Question 20.
Define law of constant proportion.
Answer:
Law of constant proportion states that, ‘In a pure chemical substance, the elements are always present in definite proportions by mass.’
Question 21.
Which organisation approves the names of elements all over the world?
Answer:
International Union of Pure and Applied Chemistry (IUPAC).
Question 22.
Name two atoms which exist as independent atoms.
Answer:
Noble gases such as argon (Ar) and helium (He) exist as independent atoms.
Question 23.
What is the number of electrons in Mg atom and Mg2+ ion?
Answer:
Mg = 12e–, Mg2+ = 10e–
Question 24.
Write atomicity of the following :
(i) Sulphur,
(ii) Phosphorus
Answer:
(i) Polyatomic,
(ii) Tetra atomic.
Question 25.
Define atomicity.
Answer:
The number of atoms present in one molecule of an element or a compound is known as its atomicity.
Question 26.
What are polyatomic ions? Give two examples.
Answer:
A group of atoms having a charge is known as polyatomicion.
Examples : (NH4)+ (SO4)2-.
Question 27.
What is the atomicity of argon?
Answer:
Monoatomic.
Question 28.
Distinguish between molecular mass and molar mass.
Answer:
The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule, whereas the mass of 1 mole of any substance is called its molar mass.
Question 29.
Define the atomic mass unit.
Answer:
The mass of 1/12th part of C-12 is equivalent to one atomic mass unit. Previous, it was denoted by symbol ‘amu’ but nowadays, it is denoted by symbol ‘u’.
Question 30.
Name the two laws of chemical combination.
Answer:
Law of conservation of mass and law of constant proportions.
Question 31.
How was the relative atomic mass determined?
Answer:
Relative atomic masses were determined by using the laws of chemical combinations and the compound formed.
Question 32.
Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Answer:
1 g of hydrogen reacts with oxygen = 8 g
3 g of hydrogen reacts with oxygen = 8 × 3 g = 24 g
Question 33.
Write the chemical symbols of two elements which are formed from the first letter of the elements’ name.
Answer:
N (Nitrogen), F (Fluorine), I (Iodine), O (Oxygen).
Short Answer Type Questions
Question 1.
What is the ratio by mass of combining elements in H2O, CO2 and NH3?
Answer:
H2O ratio by mass of combining elements = 2 : 16 = 1 : 8 (H : O)
CO2 ratio by mass of combining elements = 12 : 32 = 3 : 8 (C : 0)
NH3 ratio by mass of combining elements = 14 : 3 = 14 : 3 (N : H)
Question 2.
Calculate the ratio of the numbers of atoms for magnesium sulphide.
Answer:
Atomic mass of Mg = \(\frac{3}{24}\) = \(\frac{1}{8}\)
Atomic mass of S = \(\frac{4}{32}\) = \(\frac{1}{8}\)
Ratio of atoms in MgS = \(\frac{1}{8}\) : \(\frac{1}{8}\) or 1 : 1
Question 3.
Give the chemical name, chemical formulae for the following : Washing soda, blue vitriol, baking soda, green vitriol, oil of vitriol, soda ash, marble chips, lime water.
Answer:
Compound Name | Chemical Name | Chemical Formulae |
Washing soda | Sodium carbonate | Na2CO3.10H2O |
Blue vitriol | Copper sulphate | CuSO4.5H2O |
Baking soda | Sodium bicarbonate | NaHCO3 |
Green vitriol | Ferrous sulphate | FeSO4.7H2O |
Oil of vitriol | Sulphuric acid | H2SO4 |
Soda ash | Sodium carbonate | Na2CO3 |
Lime water | Calcium hydroxide | Ca(OH)2 |
Marble chips | Calcium carbonate | CaCO3 |
Question 4.
Find the molecular mass of the following :
H2, O2, CH4, CH3OH, CO2, HCl, Na2O
Answer:
Question 5.
Calculate the following in 5.6 g of nitrogen :
(a) Number of moles of nitrogen
(b) Number of molecules of nitrogen
(c) Number of atoms of nitrogen ‘
Answer:
(a) Molar mass of nitrogen = 14 g
5.6 g of nitrogen = 5.6/14 = 0.4 mole
(b) 28 g of nitrogen = 6.022 × 1023 molecules
5.6 g of nitrogen = 2.15 × 1022 molecules
(c) 14 g of nitrogen = 6.023 × 1023 atoms
5.6 g of nitrogen = 4.30 × 1023 atoms
Question 6.
What are ionic compounds?
Answer:
Ionic compounds are charged particles. Such compounds form by joining or losing or sharing the electron. For example : Sodium chloride is an ionic compound. Its constituent particles are positively charged sodium ion (Na+) and negatively charged chloride ion (Cl–).
Question 7.
What is the difference between hydrogen chloride and nitrogen molecule formation?
Answer:
Hydrogen chloride is molecular compound and formed by the combination of different kinds of atoms while nitrogen is diatomic molecule and formed by combination of two atoms of same kinds.
Question 8.
All elements have charged valency. Explain.
Answer:
No, all elements do not form ions thus they do not have a charge. For example – Carbon has a valency of 4 and nitrogen has a valency of 3. Non-metals are formed without a charged valency. For example – In carbon tetrachloride, carbon has valency of 4 and chlorine has a valency of 1.
Question 9.
Write the meaning of these formulae:
(i) 2O, (ii) O2, (iii) O3.
Answer:
(i) 2O = Two atoms of oxygen
(ii) O2 = One molecule of oxygen
(iii) O3 = One molecule of ozone
Question 10.
Out of these Na+, K+, Al3+ and O2-, which is iso-electronic?
Answer:
Iso-electronic means species having same number of electrons.
Ions | Atomic number of element | Atomic number of ions | Number of electron |
Na+ | 11 | 10 | 10 |
K+ | 19 | 18 | 18 |
Al3+ | 13 | 10 | 10 |
O2- | 8 | 10 | 10 |
Question 11.
Write the postulate given by the Indian philosopher Maharishi Kanad.
Answer:
Indian philosopher Maharishi Kanad postulated if we divide matter we will get smaller and smaller particles. He said that a time will come when we come across smallest particles beyond which further division will not be possible.
Question 12.
What is the significance of symbol of an element? Explain with an example.
Answer:
- The symbol of an element represents the name of the element.
- It represents one atom of the element.
- It represents a definite mass of the element.
For example :
- The symbol ‘H’ represents the element hydrogen.
- The symbol ‘H’ represents one atom of the element hydrogen.
- The symbol ‘H’ represents 1u.
Question 13.
How can Dalton’s atomic theory explain the Law of Constant Proportions?
Answer:
According to Dalton’s atomic theory, atoms of the same elements are same. Also atoms combine in whole number. This means that the atoms can combine with each other in a simple fixed ratio to form molecules.
Question 14.
What is meant by a chemical formula? Give examples.
Answer:
A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element, e.g, chemical formula of ammonia is NH3, water is H2O and carbon dioxide is CO2.
Question 15.
What is meant by a molecule? Give examples.
Answer:
A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance, e.g. molecule of oxygen is O2, ozone is O3, phosphorus is P4, sulphur is S8, etc.
Question 16.
What is the mass of :
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Answer:
(a) 1 mole of oxygen atoms = 1 × 16 = 16 g
0.2 mole of oxygen atoms = 16 g × 0.2 = 3.2 g
(b) 1 mole of water (H2O) molecules = 2 × 1g + 1 × 16g = 18g
0.5 mole of water (H2O) molecules = 18 g × 0.5 = 9.0 g
Question 17.
State two examples in each case and write their chemical formulae :
(a) Molecules having same kind of atoms only.
(b) Molecules having two different kinds of atoms.
(c) Molecules having three different kinds of atoms.
Answer:
(a) F2, Cl2, P4, S8
(b) Ammonia (NH3), Sulphur dioxide (SO2), Carbon disulphide (CS2).
(c) Calcium sulphate (CaSO4), Sodium nitrate (NaNO3).
Question 18.
Write the names of the following compounds :
(a) NiS
(b) Mg(NO3)2
(c) Na2SO4
(d) Al(NO3)3
(e) K3PO4
(f) Ca3Na2
Answer:
(a) Nickel sulphide,
(b) Magnesium nitrate,
(c) Sodium sulphate,
(d) Aluminium nitrate,
(e) Potassium phosphate,
(f) Calcium nitride.
Question 19.
Write three points of difference between an atom and a molecule.
Or
What is the difference between an atom and a molecule?
Answer:
Atom | Molecule |
1. Atom is the smallest particle of an element that takes part in a chemical reaction. | 1. Smallest particle of an element or compound that can exist independently. |
2. An atom is usually not stable itself. | 2. A molecule is usually stable itself. |
3. When similar atoms combine together in varying numbers, molecules are formed having different properties, e.g. O2, O3. | 3. When same or different molecules combine together in any numbers, a product is formed. |
Question 20.
Calculate the mass of the following :
(i) 2 moles of carbon dioxide.
(ii) 6.022 × 1023 molecules of carbon dioxide.
Answer:
(i) Molar mass of CO2 = 12 + 2 × 16g = 44g
1 mole of carbon dioxide = 44 g
2 mole of carbon dioxide = 44g × 2 = 88g
(ii) Molar mass of CO2 = 44 g
= 6.022 × 1023 molecules of carbon dioxide
Question 21.
Name the scientists whose experimentation established laws of chemical combination. Name the laws also.
Answer:
Antoine Laurent Lavoisier and Joseph L. Proust experimented and established two laws of chemical combination.
These laws are :
- Law of conservation of mass
- Law of constant proportions.
Question 22.
Give the postulates of Dalton’s atomic theory.
Answer:
Every element is composed of extremely small partides called atoms.
Atoms of a given element are identical, both in mass and properties. Different chemical elements have different kinds of atoms; in particular, their atoms have different masses.
Atoms cannot be created, destroyed or transformed into atoms of other elements. Compounds are formed when atoms of different elements combine with each other in small whole number ratios. The relative number and kinds of atoms in a given compound are constant.
Question 23.
Give two drawbacks of Dalton’s atomic theory.
Answer:
Drawbacks of Dalton’s atomic theory :
- According to modern theory, atom is not the ultimate indivisible particle of matter. Atoms are divisible, i.e. they are themselves made-up of particles (protons, electrons, neutrons, etc.).
- The assumption that the atoms of the same element have same mass does not hold good, in case of isotopes of an element.
Question 24.
What are ionic and molecular compounds? Give examples.
Answer:
Atoms of different elements join together in definite proportions to form molecules of compounds. Examples : Water, ammonia, carbon dioxide. Compounds composed of metals and non-metals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an anion and the positively charged ion is called cation, Examples : Sodium chloride, calcium oxide.
Question 25.
What are molecules? Give brief explanation of the arrangement of the constituent atoms in the molecules.
Answer:
A molecule is the smallest particle of an element or compound which is stable in normal conditions.
And it can freely show all the properties of that element or compound. It may be made up of one, two or more atoms. Molecule with one atom called monoatomic, e.g. helium, neon, etc.
Molecule with two atoms called diatomic, e.g. Cl2, O2. Similarly, there are molecules containing three atoms (CO2), four atoms (P4) and so on.
Long Answer Type Questions
Question 1.
(a) What is the use of mole concept?
Answer:
Applications of mole concept:
(i) We can calculate the number of basic particles from the number of moles as the number of moles of a substance is directly proportional to the number of elementary particles.
(ii) One mole of a gas occupies 22.4 litres at 273K.
(iii) One mole of any gas occupies the same volume at same pressure and temperature.
(iv) One mole is equal to 6.022 × 1023 atoms. So, we can calculate the absolute masses of atoms and molecules.
(b) Define the term gram atom. How is it related to mole and Avogadro number?
Answer:
The atomic mass of an element expressed in grams is called gram atomic mass.
One gram atom of any element contains 6.022 × 1023 atoms of the element. It is equal to one mole of atoms.
One gram atomic mass = 6.022 × 1023 atoms = 1 mole
Question 2.
Give symbol and valency of: Potassium, Barium, Aluminium, Calcium, Cobalt, Fluorine, Lead, Zinc, Iodine, Sulphide.
Answer:
Name of Ion | Symbol | Valency |
Potassium | K+ | +1 |
Barium | Ba2+ | +2 |
Aluminium | Al3+ | +3 |
Calcium | Ca2+ | +2 |
Cobalt | Co2+ | +2 |
Fluorine | F– | -1 |
Lead | Pb+ | +1 |
Zinc | Zn2+ | +2 |
Iodine | I– | -1 |
Sulphide | S2- | -2 |
Question 3.
Write formula for the following :
(a) Zinc sulphate,
(b) Methane,
(c) Ammonium carbonate.
Answer:
(a) Zinc sulphate
Thus, Zn(SO4)2 and finally = ZnSO4
(b) Methane
Thus, finally = CH4
(c) Ammonium carbonate
Thus, finally = (NH4)2CO3
Question 4.
(a) Explain the law of multiple proportions.
(b) Explain the form of atoms in a solid.
Answer:
(a) According to law of multiple proportions, when two elements combine to make one or more compounds then the ratio of weights of these element remain in fixed ratio to one another. For example : Hydrogen and oxygen combine to form water (H2O) and hydrogen peroxide (H2O2) under different conditions. 2 grams of hydrogen combines with 16 grams of oxygen in case of water while 2 grams of hydrogen combines with 32 grams Of oxygen to form hydrogen peroxide. Now, the weights of oxygen combine with a fixed weight of hydrogen in water and hydrogen peroxide respectively are 16 and 32 which are in simple ratio of 16 : 32 or 1 : 2.
(b) A solid element is a cluster of atoms. The property of solid does not depend on a single atom but on cluster of atoms. For example : Diamond and graphite though both are composed of carbon atoms but due to different arrangements of carbon atoms in these. They are different in physical and chemical properties.
Question 5.
What is the mass of:
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2SO3)?
Answer:
(a) 1 mole of nitrogen atoms
= 1 × gram atomic mass of nitrogen atom
= 1 × 14 g = 14 g
(b) 4 moles of aluminium atoms
= 4 × gram atomic mass of aluminium atoms
= 4 × 27 g = 108 g
(c) 10 moles of sodium sulphite (Na2SO3)
= 10 (2 × gram atomic mass of Na + 1 × gram atomic mass of sulphur + 3 × gram atomic mass of oxygen)
= 10 (2 × 23 g + 1 × 32 g + 3 × 16 g)
= 10 (46 g + 32 g + 48 g)
= 10 × 126 g = 1260 g
Question 6.
Explain the law of conservation of mass with an experiment.
Answer:
According to the law of conservation of mass, during any physical and chemical change, the matter is neither created nor destroyed. It may change from one form to another. It can be described through an activity.
Experiment: Prepare a 5% solution of barium chloride and sodium sulphate each in 10 mL water. Take a little amount of solution of sodium sulphate in a conical flask and some solution of barium chloride in ignition tube. Hang the ignition tube in the flask carefully and see that solution do not get mixed. Put a cork on the flask. Weigh the flask with its contents carefully. Now tilt and swirl the flask, so that the solution get mixed. The reaction takes place and a white precipitate is obtained. The tube is weighed after the reaction has taken place. The mass of the tube is found to be exactly the same as the mass obtained before inverting the tube. This experiment verifies the law of conservation of mass.
Fig.: Ignition tube containing solution of barium chloride, dipped in a conical flask containing solution of sodium sulphate
Question 7.
Represent the relation between mole, Avagadro number and mass by a diagram.
Answer:
Numerical Questions
Question 1.
Glucose has the molecular formula C6H12O6. Calculate :
(a) Its molecular mass.
(b) The number of atoms in one molecule of glucose.
(c) The number of gram molecule in 18 g of glucose.
Answer:
(a) Molecular mass of C6H12O6 = (6 × 12u) + (12 × 1u) + (6 × 16u)
= 72u + 12u + 96u = 180u
(b) The number of atoms in one molecule of C6H12O6
= 6 atoms of C + 12 atoms of H + 6 atoms of O
= 6 + 12 + 6 = 24 atoms
(c)
Question 2.
What is the mass of (i) 2.5 moles of CO2 and (ii) 1 mole of water?
Answer:
(i) 1 mole of CO2 = Molecular mass expressed in grams
= 1 × 44 g
2.5 moles of CO2 = 2.5 × 44 = 110 g
(ii) Mass of the substance = Moles of substance × Molecular mass in grams
Mass of water = 1 × 18 g = 18 g
Question 3.
Calculate the number of H2O molecules in one drop of water having a mass of 0.05 g.
Answer:
Number of moles of H2O in 0.05 g of water = \(\frac{0.05 \mathrm{~g}}{18 \mathrm{~g}}\) (molar mass of water = 18)
= \(\frac{0.05 \mathrm{~g} \times 6.022 \times 10^{23}}{18 \mathrm{~g}}\)
= 1.673 × 1023 molecules
Question 4.
What is the mass percentage of different elements in calcium carbonate? (Atomic mass : Ca = 40, C = 12, O = 16)
Answer:
Molecular mass of CaCO3
= Atomic Mass of Ca + Atomic Mass of C + 3 x Atomic Mass of O
= 40 + 12 + 3 × 16 = 100
Question 5.
The mass of any single atom X is 3.05 × 10-22 g. What is its atomic weight? Name the possible element.
Answer:
1 mole = atomic mass = 6.022 × 1023 atoms
Now, mass of one atom of X = 3.05 × 10-22 g
Mass of 6.022 × 1023 atoms of X = 3.05 × 10-22 × 6.022 × 1023 g
= 183.7 g
This element could be tungsten.
Question 6.
50 g of 10% lead nitrate is mixed with 50 g of 10% sodium chloride in a closed vessel. It was found after reaction that 6.83 g of lead chloride was precipitated. Besides, the reaction mixture contained 90 g water and sodium nitrate. Calculate the amount of sodium nitrate formed.
Answer:
50 g of 10% lead nitrate = 5 g lead nitrate + 45 g water
50 g of 10% sodium chloride = 5 g sodium chloride + 45 g water
Total content before reaction = 5 + 5 + 90 = 100
Total content eifter reaction = 90 g
Amount of precipitate = 6.83 g
According to law of conservation,
Total mass of reaction mixture = 100 g
Amount of sodium nitrate = 100 – (90 + 6.83) = 3.17 g
Question 7.
The mass of one molecule of a substance is 4.65 × 1023 grams. What is its molecular mass?
Answer:
Mass of 1 molecule of a substance = 4.65 × 1023 grams
Mass of 6.023 × 10-23 molecules of a substemce = 4.65 × 1023 × 6.023 × 10-23
= 28 g
Molecular mass of the substance = 28 g
Question 8.
When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 5.00 g of magnesium oxide is produced. What mass of magnesium oxide will be formed when 3.00 g magnesium is burnt in 5.00 g of oxygen? Which law of chemical combination will govern your answer? State the law.
Answer:
When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 5.00 g of magnesium oxide is produced. It means magnesium and oxygen are combined in the ratio of 3 : 2 to form magnesium oxide.
Thus, when 3.00 g of magnesium is burnt in 5.00 g of oxygen, 5.00 g of magnesium oxide will be formed and the remaining oxygen will be left unused. It is governed by law of definite proportions.
It states that in a chemical substance, the elements are always present in definite proportions by mass.
Question 9.
A sample of ethane (C2H6) gas has the same mass as 1.5 × 1020 molecules of methane (CH4). How many (C2H6) molecules does the sample of gas contain?
Answer:
Question 10.
(a) Calculate the number of molecules of SO2 present in 44 g of it.
(b) If one mole of oxygen atoms weighs 16 grams, find the mass of one atom of oxygen in grams.
Answer:
(a) Molecular mass of SO2 = Atomic mass of S + 2 × Atomic mass of 0
= 32 + 2 × 16 = 64u
Molar mass = 64 g
(b) One mole of oxygen contains 6.022 x 1023 atoms of oxygen
Mass of one atom of oxygen = 16/6.022 × 1023 g
= 2.66 × 10-23 g
Question 11.
Sodium is represented as 23Na11.
(a) What is its atomic mass?
(b) Write its gram atomic mass.
(c) How many atoms of Na will be there in 11.5 g of the sample?
Answer:
(a) Atomic m ass = 23u
(b) Gram atomic mass = 23 g
(c) Given mass = 11.5 g Molar mass = 23 g
= 11.5 × 6.022 × 1023/23 = 3.011 × 1023 atoms
Question 12.
A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?
Answer:
One gram of gold sample will contain \(\frac{90}{100}\) = 0.9 g of gold
= \(\frac{0.9}{197}\) = 0.0046
One mole of gold contains NA atoms = 6.022 × 1023
∴ 0.0046 mole of gold will contain = 0.0046 × 6.022 × 1023
= 2.77 × 1021
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