Laws of Chemical Combination (Conservation of Mass, Constant Proportions) MCQs

Q1. In a flask 10.6 g of sodium carbonate is mixed with 7.3 g of hydrochloric acid. The products are sodium chloride, water and carbon dioxide. If 11.7 g of sodium chloride and 1.8 g of water are collected, what mass of carbon dioxide escaped?

• A.13.5 g
• B.17.9 g
• C.4.4 g
• D.6.2 g

Answer: C. 4.4 g

By conservation of mass, total mass of reactants = total mass of products. Reactant mass = 17.9 g. Known other masses = 13.5 g. So CO2 = 17.9 - 13.5 = 4.4 g.

Q2. In a closed container 12.0 g of carbon is burned with 32.0 g of oxygen to give carbon dioxide as the only product. If the reaction is complete, what mass of carbon dioxide is formed?

• A.22 g
• B.44 g
• C.12 g
• D.32 g

Answer: B. 44 g

By conservation of mass, total mass of reactants = total mass of products. Reactant mass = 44 g. Known other masses = 0 g. So CO2 = 44 - 0 = 44 g.

Q3. When 25.0 g of marble chips (calcium carbonate) react with 18.25 g of hydrochloric acid in a flask fitted with a balloon, the products are 27.75 g of calcium chloride, 4.5 g of water and carbon dioxide. What mass of carbon dioxide inflates the balloon?

• A.15.5 g
• B.11 g
• C.32.25 g
• D.43.25 g

Answer: B. 11 g

By conservation of mass, total mass of reactants = total mass of products. Reactant mass = 43.25 g. Known other masses = 32.25 g. So CO2 = 43.25 - 32.25 = 11 g.