The $Ka$ for weak acid that form buffer system with its conigate base was determined by mixing same concentrations of acid and the conjugate base to give buffer with pH equale $6$ is $?$

a$.6\mathrm{.}0$

b$.6{10}^{-6}$

c$.1{10}^{-6}$

d$.6\mathrm{.}4$

e$.6\mathrm{.}6$

A buffer system is set up with $[HA]=2[A^{-}].$ If pKa$=6\mathrm{.}5,$ what is the $pH$ of the buffer?

a$.6\mathrm{.}8$

b$.3\mathrm{.}5$

c$.5\mathrm{.}8$

d$.7\mathrm{.}0$

e$.7\mathrm{.}5$

$11.$The $ppm$ concentration of $C{l}^{-}$ion in $100ml$ solution of mixture of $0\mathrm{.}01MMgC{l}_2$ and $0\mathrm{.}1MHCl$ is $($MW of Mg $=24\mathrm{.}305\frac{g}{m}ol$ and for $Cl=35\mathrm{.}45\frac{g}{m}ol)$ is :$-$

a$.243$ ppm

b$.4254ppm$

c$.355ppm$

d$.950ppm$