A capacitor $C_{1}$ of capacitance $5\ \mu$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 $C_{2}$ of capacitance $10\ \mu$F as shown in figure. When the switch is closed charge flows between the capacitors. At equilibrium, the charge on the capacitor $C_{2}$ is \_\_\_\_ $\mu$C.