A solid, Q, reacts with a solution, P, to form a new solution, R, and a gas, Z.
A student carries out the reaction in two flasks. Before each reaction, P and Q are kept apart inside the flask while the initial balance reading is recorded. The flask is then tipped to mix the reactants. No liquid or solid leaves either flask.
| Experiment | Flask | Balance reading before reaction / g\text{g}g | Balance reading after reaction / g\text{g}g |
|---|---|---|---|
| 1 | open | 73.84 | 72.46 |
| 2 | sealed | 81.27 | not recorded |
Calculate the decrease in the balance reading in experiment 1.
Explain why the balance reading decreases in experiment 1 even though mass is conserved in the reaction.
State the balance reading expected after the reaction in experiment 2. Explain your answer.
134 exam-style questions on OCR GCSE Chemistry 3.1 Introducing chemical reactions, covering 3.1.1 Writing formulae and balanced chemical equations, 3.1.2 Formulae of compounds from ions, and state symbols, 3.1.3 Conservation of mass and observed changes in mass, 3.1.4 Half equations and balanced ionic equations, 3.1.5 The mole and the Avogadro constant, and 3.1.6 Stoichiometry, limiting reactants and reacting masses. Each one has a worked solution and a mark scheme showing where the marks go.