A technician investigates conservation of mass in two experiments.
Experiment A: Copper carbonate is heated until it has completely decomposed. Copper oxide and carbon dioxide are formed.
CuCO3(s)→CuO(s)+CO2(g)\mathrm{CuCO_3(s) \rightarrow CuO(s) + CO_2(g)}CuCO3(s)→CuO(s)+CO2(g)
Experiment B: Dilute hydrochloric acid is placed in a conical flask and magnesium ribbon is placed inside a balloon. The balloon is fitted tightly over the flask. The magnesium is tipped into the acid without removing the balloon. The balloon remains attached throughout the reaction.
Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\mathrm{Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)}Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)
| Experiment | Measurement before the reaction | Measurement after the reaction |
|---|---|---|
| A | Mass of copper carbonate: 12.35 g | Mass of copper oxide: 7.95 g |
| B | Balance reading for flask, balloon and contents: 158.37 g | Balance reading for flask, balloon and contents: 158.37 g |
Calculate the mass of carbon dioxide made in Experiment A.
Explain why the mass of copper oxide in Experiment A is less than the starting mass of copper carbonate and why the balance reading in Experiment B stays the same.
Use ideas about particles and conservation of mass.
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.