Question

7. Determine the volume (in mL) of a 2.0 M NaOH solution must be added to 300. mL of 0.10 M NaH2PO4 to make buffer solution with a pH of 7.30. (pKal of H2PO4 is 7.21)

Answer 1

Let V be the volume of NaOH solution required.

From Henderson Hasselbalch equation:

$\small pH=pKa+\log \frac{[HPO_{4}^{2-}]}{[H_{2}PO_{4}^{-}]}$

$\small 7.30=7.21+\log \frac{((300\times 0.1)+(2.V))}{((300\times 0.1)-(2.V))}$

$\small \frac{((300\times 0.1)+(2.V))}{((300\times 0.1)-(2.V))}=10^{0.09}$

$\small \frac{30+2V}{30-2V}=1.2303$

$\small 30+2V=1.2303\times (30-2V)$

Solving for V, we get V = 1.55 mL

• That is the volume of NaOH required is 1.55 mL