A student investigates the mass changes in two reactions using a top-pan balance.
In reaction 1, the student places a flask containing sodium chloride solution and a small tube of silver nitrate solution on the balance. The flask is sealed with a bung. The student tips the flask so that the two solutions mix. A white solid forms.
NaCl(aq)+AgNO3(aq)→AgCl(s)+NaNO3(aq)\mathrm{NaCl(aq)} + \mathrm{AgNO_{3}(aq)} \rightarrow \mathrm{AgCl(s)} + \mathrm{NaNO_{3}(aq)}NaCl(aq)+AgNO3(aq)→AgCl(s)+NaNO3(aq)
State the law of conservation of mass.
The total mass of the flask and its contents before mixing is 162.50 g. State the total mass after the reaction.
Give the name of the white solid formed.
In reaction 2, the student adds marble chips (calcium carbonate) to dilute hydrochloric acid in an open conical flask on the balance.
| Time | Mass of flask and contents / g |
|---|---|
| at the start | 105.60 |
| at the end | 104.72 |
Calculate the decrease in mass.
Explain why the mass of the flask and contents decreases in reaction 2.
Suggest one change the student could make to reaction 2 to show that mass is conserved.
55 exam-style questions on Edexcel GCSE Chemistry 2.6 Calculations involving masses, covering 2.6.1 Relative formula mass and percentage composition, 2.6.2 Empirical and molecular formulae, 2.6.3 Conservation of mass, 2.6.4 Reacting masses and concentration in g dm⁻³, 2.6.5 The mole and the Avogadro constant, and 2.6.6 Limiting reactants and stoichiometry. Each one has a worked solution and a mark scheme showing where the marks go.