Laws of Chemical Combination - UNSOLVED PRACTICE SET

Q7. State the Law of Conservation of Mass. Explain it with a simple example from a reaction you might observe in your school laboratory.

Q8. State the Law of Definite Proportions. A sample of water from a river in Kerala and a sample of water from a well in Rajasthan both contain hydrogen and oxygen. What can you say about the mass ratio of hydrogen to oxygen in both samples?

Q9. Your chemistry teacher burns 12 g of magnesium ribbon in air, and 20 g of magnesium oxide is obtained. Using the Law of Conservation of Mass, calculate how much oxygen combined with the magnesium. Show your reasoning.

Q10. State the Law of Multiple Proportions. Carbon forms two oxides — carbon monoxide (CO) and carbon dioxide (CO₂). If 3 g of carbon combines with 4 g of oxygen in CO, how much oxygen will combine with the same mass of carbon in CO₂? What is the ratio of the two masses of oxygen?

Q11. Why do the Laws of Chemical Combination provide strong evidence for the atomic theory? Explain in 2–3 sentences.

Q12. A student performs an experiment where 10 g of calcium carbonate is heated strongly. The products are 5.6 g of calcium oxide and 4.4 g of carbon dioxide. Verify whether this data supports the Law of Conservation of Mass.

Q13. Explain the three main Laws of Chemical Combination — Conservation of Mass, Definite Proportions, and Multiple Proportions — with one example for each. How do these laws collectively support the idea that matter is composed of atoms?

Q14. A chemist analyses three different samples of ammonia (NH₃) obtained from different sources:

- Sample A: 2.8 g nitrogen combines with 0.6 g hydrogen  

- Sample B: 5.6 g nitrogen combines with 1.2 g hydrogen  

- Sample C: 14 g nitrogen combines with 3 g hydrogen

(a) Calculate the ratio of nitrogen to hydrogen in each sample.  

(b) Do these samples support the Law of Definite Proportions? Explain.  

(c) What conclusion can you draw about the composition of ammonia?

Q15. "The Laws of Chemical Combination are not just rules in a textbook — they govern everything from cooking in your kitchen to rocket science." Discuss this statement by giving two real-life examples where the Law of Conservation of Mass and the Law of Definite Proportions play a role, even if we don't notice them.

Q16. In a school laboratory experiment, a student mixes 15 g of sodium sulphate (Na₂SO₄) with 20 g of barium chloride (BaCl₂). The reaction produces barium sulphate (BaSO₄) and sodium chloride (NaCl).

(a) Write the balanced chemical equation for this reaction.  

(b) If 23.3 g of barium sulphate is obtained, calculate the mass of sodium chloride formed using the Law of Conservation of Mass.  

(c) Verify your answer by calculating the total mass of reactants and products.  

(d) What would happen if the student used only 10 g of barium chloride instead? Would the total mass still be conserved?

Q17. Sulphur forms two oxides — sulphur dioxide (SO₂) and sulphur trioxide (SO₃).

(a) In SO₂, 32 g of sulphur combines with 32 g of oxygen. Calculate the mass of oxygen that combines with 32 g of sulphur in SO₃.  

(b) Find the ratio of the masses of oxygen combining with the same mass of sulphur in SO₂ and SO₃.  

(c) Does this ratio support the Law of Multiple Proportions? Justify your answer with calculations.  

(d) Express this ratio in its simplest whole number form.

Q18. A baker uses baking soda (NaHCO₃) in a cake recipe. When heated, baking soda decomposes as follows:

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

(a) If the baker uses 16.8 g of baking soda, and the products obtained are 10.6 g of sodium carbonate, 1.8 g of water, and 4.4 g of carbon dioxide, verify the Law of Conservation of Mass.  

(b) Calculate the percentage by mass of each product.  

(c) If the baker accidentally uses 25.2 g of baking soda, what total mass of products should she expect?  

(d) Why is it important for a baker to understand this law, even if they don't know chemistry?