Measuring acid concentration

In rate of reaction graphs it is usual to show the concentration of a reactant decreasing. In this experiment, therefore, we need to have a measure of the concentration of the hydrochloric acid to plot on our graph. We use either (m_final - m_t) or (V_final - V_t) as a measure of the concentration of the hydrochloric acid. The reason for this has been asked on exams before.

m_final and V_final are the final mass loss and final gas volume respectively. m_t and V_t are the mass loss and gas volume at some time, t. The theory behind this is the same for both these cases. If you consider the volume case, we start with zero volume and it rises to a maximum, V_final. However, we want the concentration of the hydrochloric acid, which start as a maximum and ends up at zero (when it has all been used up by the reaction). V_final-V_t is just a way of changing the rising volume into the decreasing acid concentration.

Graph showing the relationship between increasing gas volume and decreasing acid concentration