A biochemical assay requires of a phosphate buffer at pH ( ) that can neutralize of…

CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases

Problem

A biochemical assay requires

750\,\text{mL}

of a phosphate buffer at pH

7.40

(

pK_{a_2} = 7.21

) that can neutralize

0.030\,\text{mol}

of added strong acid. You have

0.200\,\text{M}

stock solutions of

H_2PO_4^-

(acid form) and

HPO_4^{2-}

(base form). Which combination of stock solution volumes will satisfy both the pH and capacity criteria after dilution to

750\,\text{mL}

? (a)

75\,\text{mL}

HPO_4^{2-}

and

48\,\text{mL}

H_2PO_4^-

, then water to

750\,\text{mL}

(b)

150\,\text{mL}

HPO_4^{2-}

and

97\,\text{mL}

H_2PO_4^-

, then water to

750\,\text{mL}

(c)

97\,\text{mL}

HPO_4^{2-}

and

150\,\text{mL}

H_2PO_4^-

, then water to

750\,\text{mL}

(d)

200\,\text{mL}

HPO_4^{2-}

and

40\,\text{mL}

H_2PO_4^-

, then water to

750\,\text{mL}