Calculate the $pH$ of the following buffers:
a)$0.050\;M$ $CH_3COOH$ and $0.050\;M$ $CH_3COONa$ solution
b)$0.150\;M$ $HCOOH$ and $0.250\; M$ $HCOONa$ solution
c)$0.050\;M$ $HNO_2$ and $0.20\;M$ $NaNO_2$ solution
d) $0.10\;M$ lactic acid and $0.050\;M$ sodium lactate solution