#### Answer

2.2 V

#### Work Step by Step

Energy $E=Power\times time=0.50\,W\times5.0\,s=2.5\,J$
The expression connecting energy with the terminal voltage is
$E=\frac{1}{2}CV^{2}$
$\implies V=\sqrt {\frac{2E}{C}}=\sqrt {\frac{2\times2.5\,J}{1.0\,F}}=2.2\,V$