Phosphoric acid is a triprotic acid with the $p{K}_a$ values $p{K}_{a1}=2\mathrm{.}148,p{K}_{a2}=7\mathrm{.}198,$ and $p{K}_{a3}=12\mathrm{.}375.$

You wish to prepare $1\mathrm{.}000L$ of a $0\mathrm{.}0400M$ phosphate buffer at $pH7\mathrm{.}640.$ To do this, you choose to mix the two salt forms

involved in the second ionization, $Na{H}_{2}P{O}_4$ and $N{a}_{2}HP{O}_4,$ in a $1\mathrm{.}000L$ volumetric flask and add water to the mark. What mass

of each salt will you add to the mixture?

What other combination of phosphoric acid and its salts could be mixed to prepare this buffer?

$H_{3}P{O}_4$ and $N{a}_{2}HP{O}_4$

$H_{3}P{O}_4$ and $Na{H}_{2}P{O}_4$