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Chapter 3 Atoms And Molecules Solutions

Question - 11 : - Calculate the molecular masses of H2, O2 Cl2,CO2, CH4 C2H6, C2H4 NH3, CH3OH.

Answer - 11 : -


Question - 12 : - Calculate the formula unit masses of ZnO, Na2O, K2C03.Given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u,C = 12 u and,0= 16 u.

Answer - 12 : - Formula unit mass of ZnO = atomic mass of Zn + atomicmass of O

Question - 13 : - If one mole of carbon atoms weighs 12 gram, what isthe mass (in gram) of 1 atom of carbon?

Answer - 13 : -


Question - 14 : - Which has more number of atoms, 100 grams of sodium or100 grams of iron (given atomic mass of Na = 23 u, Fe = 56 u)?

Answer - 14 : -

Question - 15 : - A 0.24 g sample of compound of oxygen and boron wasfound by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculatethe percentage composition of the compound by weight.

Answer - 15 : - We know that, % of any element in a

Question - 16 : - When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.0 gof carbon dioxide is produced. What mass of carbon dioxide will be formed when3.0 g of carbon is burnt in 50.00 g of oxygen? Which law of chemicalcombination will govern your answer?

Answer - 16 : -


Total mass of reactants = total mass of products Hence, the lawof conservation of mass is proved.
Further, it also shows carbon dioxide contains carbon and oxygen in a fixedratio by mass, which is 3 : 8. Thus, it also proves the law of constantproportions. 3 g of carbon must also combine with 8 g of oxygen only. Thismeans that (50 – 8) = 42 g of oxygen will remain un reacted.

Question - 17 : - What are polyatomic ions? Give examples.

Answer - 17 : - A polyatomic ion is a group of atoms carrying positiveor negative charge.

Question - 18 : - Write the chemical formulae of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate

Answer - 18 : - (a) Magnesium chloride


(b) Calcium oxide

(c) Copper nitrate

(d) Aluminium chloride

(e) Calcium carbonate

Question - 19 : - Give the names of the elements present in thefollowing compounds:
(a) Quicklime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate

Answer - 19 : -

(a) Quick lime is CaO. Elements present are calcium and oxygen.
(b) Hydrogen bromide is HBr. Elements present are hydrogen and bromine.
(c) Baking powder is NaHC03.Elements present are sodium, hydrogen, carbon and oxygen.
(d) Potassium sulphate is K2S04. Elements present arepotassium, sulphur and oxygen.

Question - 20 : - Calculate the molar mass of the following substances:
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (atomicmass of phosphorus = 31)
(d) Hydrochloric acid, HCI
(e) Nitric acid, HN03

Answer - 20 : - (a) Molar mass of C2H2


 (b) Molar mass of S = 8 × atomic mass of s = (8 × 32)u= 256 u

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