CAIE A Level Chemistry 9701 · Atoms, molecules and stoichiometry
A student determines the concentration of a sample of hydrochloric acid by adding an excess of calcium carbonate to 25.0 cm³ of the acid and collecting the carbon dioxide produced in a gas syringe.
Explain why the calcium carbonate must be in excess and why the volume of gas must be measured at a known temperature and pressure.
4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
Written by Markquill to follow the Cambridge International syllabus. Markquill is independent and is not affiliated with or endorsed by Cambridge International.