5 g zinc are added to 400 cm 3 or 0.5 mol. dm-3 hydrochloric acid. To investigate the average rate of the reaction, the change in the mass of the flask containing the since and the acid is measured by placing the flask on a direct reading balance. The reading on the balance shows that there is a decrease in mass during the reaction. The reaction which takes place is given by the following equation: Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)