Q4. 50 mL of nitrogen is mixed with 200 mL of hydrogen, and they react completely as far as possible: N2(g) + 3H2(g) → 2NH3(g). All volumes are measured at the same temperature and pressure. What volume of ammonia forms, and which gas is left over?
50 mL of N2 needs 3 × 50 = 150 mL of H2 and gives 2 × 50 = 100 mL of NH3. Hydrogen is in excess: 200 − 150 = 50 mL of H2 is left.
Q5. Sulphur dioxide is 50.0% sulphur by mass and sulphur trioxide is 40.0% sulphur. Show that these data illustrate the law of multiple proportions.
In SO2, 50.0 g of S combines with 50.0 g of O: 1.00 g of O per gram of S. In SO3, 40.0 g of S combines with 60.0 g of O: 1.50 g of O per gram of S. For the same mass of sulphur, the masses of oxygen are in the ratio 1.00 : 1.50 = 2 : 3, a small whole-number ratio.
Q6. 1.20 g of magnesium burns in air to give 2.00 g of magnesium oxide. What mass of oxygen combined with it?
By conservation of mass, 2.00 − 1.20 = 0.80 g of oxygen.
Q7. 20 mL of H2 and 20 mL of O2 are exploded together and cooled to room temperature. What volume of gas remains? Compare volumes at the same initial and final temperature and pressure, and neglect residual water vapour.
2H2 + O2 → 2H2O: 20 mL of H2 uses 10 mL of O2. The water condenses to a liquid of negligible volume, so 10 mL of O2 remains.