Exercises: Solutions

1. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

   Solution:

   • Percentage of boron = $\frac{0.096}{0.24} \times 100 = 40\%$
   • Percentage of oxygen = $\frac{0.144}{0.24} \times 100 = 60\%$

   Answer:

   • Boron: 40%
   • Oxygen: 60%

2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

   Solution:
   According to the Law of Constant Proportions, a fixed ratio of carbon and oxygen will always form a certain amount of carbon dioxide. Here, 3.0 g of carbon reacts with exactly 8.00 g of oxygen to form 11.00 g of carbon dioxide. Any excess oxygen (50.00 g in this case) will not change the amount of carbon dioxide formed.

   Answer:

   • Mass of carbon dioxide produced = 11.00 g
   • Governed by Law of Constant Proportions.

3. What are polyatomic ions? Give examples.

   Solution:
   Polyatomic ions are ions that consist of two or more atoms covalently bonded together, carrying a net positive or negative charge.

   Examples:

   • Ammonium, $\text{NH}_4^+$
   • Sulfate, $\text{SO}_4^{2-}$
   • Nitrate, $\text{NO}_3^-$

4. Write the chemical formulae of the following.

   • (a) Magnesium chloride: MgCl₂
   • (b) Calcium oxide: CaO
   • (c) Copper nitrate: Cu(NO₃)₂
   • (d) Aluminium chloride: AlCl₃
   • (e) Calcium carbonate: CaCO₃

5. Give the names of the elements present in the following compounds.

   • (a) Quick lime: Calcium, Oxygen
   • (b) Hydrogen bromide: Hydrogen, Bromine
   • (c) Baking powder: Sodium, Hydrogen, Carbon, Oxygen
   • (d) Potassium sulphate: Potassium, Sulfur, Oxygen

6. Calculate the molar mass of the following substances.

   • (a) Ethyne, $\text{C}_2\text{H}_2$
     $\text{Molar mass} = (2 \times 12) + (2 \times 1) = 24 + 2 = 26 \, \text{g/mol}$

   • (b) Sulfur molecule, $\text{S}_8$
     $\text{Molar mass} = 8 \times 32 = 256 \, \text{g/mol}$

   • (c) Phosphorus molecule, $\text{P}_4$
     $\text{Molar mass} = 4 \times 31 = 124 \, \text{g/mol}$

   • (d) Hydrochloric acid, $\text{HCl}$
     $\text{Molar mass} = 1 + 35.5 = 36.5 \, \text{g/mol}$

   • (e) Nitric acid, $\text{HNO}_3$
     $\text{Molar mass} = 1 + 14 + (3 \times 16) = 1 + 14 + 48 = 63 \, \text{g/mol}$