A calorimeter of mass $50\mathrm{g}$ and specific heat capacity $0.42\mathrm{J}{\mathrm{g}}^{-1}{}^{\circ }{\mathrm{C}}^{-1}$ contains some mass of water at ${20}^{\circ }\mathrm{C}$. A metal piece of mass $20\mathrm{g}$ at ${100}^{\circ }\mathrm{C}$ is dropped into the calorimeter. After stirring, the final temperature of the mixture is found to be ${22}^{\circ }\mathrm{C}$. Find the mass of water used in the calorimeter.
[specific heat capacity of the metal piece $=$ $0.3{\mathrm{Jg}}^{-1}{}^{\circ }\mathrm{C}-1$
specific heat capacity of water $=4.2\mathrm{J}{\mathrm{g}}^{-1}$ ${}^{\circ }{\mathrm{C}}^{-1}]$