A capacitor $\mathrm{C}_1$ of capacitance $5 \mu \mathrm{~F}$ is charged to a potential of 30 V using a battery. The battery is then removed and the charged capacitor is connected to an uncharged capacitor $\mathrm{C}_2$ of capacitance $10 \mu \mathrm{~F}$ as shown in figure. When the switch is closed charge flows between the capacitors. At equilibrium, the charge on the capacitor $\mathrm{C}_2$ is $\_\_\_\_$ $\mu \mathrm{C}$.