Absorption of a trace gas by a liquid droplet is simulated by solving the coupled time-dependent diffusion equations for gas and liquid phase. A comprehensive numerical model and a new approximate analytical solution are compared with previous simplified expressions which describe the dependence of gas uptake on solubility, diffusion and mass accommodation. The model and the novel formula are suited for the evaluation of laboratory droplet experiments. Specifically, we re-evaluate measured uptake data of HCl on aqueous sulphuric acid droplets. Our treatment corroborates the earlier finding that the mass accommodation coefficient gamma (0) of HCl on aqueous H2SO4 is about 0.15, but this value might be influenced by the presence of forced convection and the Stefan flows as well as the inaccuracy in the Henry's law coefficient of HCl.