A diode is connected to 220 V (rms) ac in series with a capacitor as shown in figure. the voltage across the capacitor is

The diode D will conduct for positive half cycle of ac supply because this is forward biased.For negative half cycle of ac supply, this is reverse biased and does not conduct. So, output would be half wave rectified and for half wave rectified output

${\mathrm{V}}_{\mathrm{rms}}=\frac{{\mathrm{V}}_{\mathrm{o}}}{2}=\frac{200\sqrt{2}}{2}=\frac{200}{\sqrt{2}}\mathrm{V}$