Question

Create a schematic diagram on how to prepare a Phosphate Buffer NaH2PO4 - K2HPO4 with a final volume of 500. mL, concentration of 0.10M with a pH of 7.20 from 0.20 M NaH2PO4 and solid K2HPO4.
Note: Ka = 6.3 x 10-8


Reminders: Show all calculations before preparing the schematic diagram.
For solid K2HPO4, calculate for the mass (based from calculated moles of salt) rather than the volume since this is a solid substance.

Answer 1

H₂PO₄^- acts as acid and HPO₄^2- acts as salt.

$pK_a = -log(K_a)$

$pK_a = -log(6.3\times 10^{-8}) = 7.2$

The required pH = 7.20

$pH = pK_a + log\frac{[K_2HPO_4]}{[NaH_2PO_4]}$

Since pH = pKa the salt and acid concentrations are equal.

[NaH₂PO₄] = [K₂HPO₄]

Given the total concentration = 0.10 M

Therefore the concentration of [NaH₂PO₄] = 0.05 M

The stock solution of NaH₂PO₄ = 0.2 M

Therefore the volume of stock solution required $V = 500 \times \frac{0.05 }{ 0.2} = 125 \;mL$

We need 125 mL of 0.2 M NaH₂PO₄ solution.

The concentration of [K₂HPO₄] = 0.05 M

The total volume of the buffer solution V = 500 mL

Therefore moles of K₂HPO₄ in the buffer

$n = 500\; mL \times \frac{0.05\; mol }{ 1000 \;mL} = 0.025 \;moles$

M(K₂HPO₄) = 174 g/mol

m=n×M

$m(K_2HPO_4) = 0.025\; mol \times 174\;g/mol = 4.35 \;g$