Phosphate Buffer Preparation :Describe how would you prepare a phosphate buffer solution (100.0 mL, 0.100 M, pH 7.00)?

Question

H2PO4− (aq) + H2O (l) ⇌ HPO42− (aq) + H3O+ (aq) pKa2 = 7.20

Question: Describe how would you prepare a phosphate buffer solution (100.0 mL, 0.100 M, pH 7.00)?

Available Stock Solution: Concentrated H3PO4 (14.8 M)

Available Solids: NaH2PO4 ∙ 2H2O, anhydrous Na2HPO4, anhydrous Na3PO4

Write the information required in the table below. 4 pts

	Choice of Reagent:	Amount (g or mL):
Acidic Component	NaH2PO4 ∙ 2H2O	0.957 g
Basic Component	anhydrous Na2HPO4	0.549 g

To prepare the buffer solution dissolve 0.957g of the acidic component and 0.549g of the basic component with distilled water and dilute to mark in a 100mL vol. flask. Adjust pH if necessary before diluting.

(Ps: are all my provided answers correct?)

Answer 1

You've correctly identified the acidic and basic components, but I get a different answer than you for the masses of each. Since you didn't include your calculations, I'm not sure why we differ. My approach is shown below, but please do check the math.

pH = pKa + log [HPO₄^2-]/[H₂PO₄^-]

7.00 = 7.20 + log [HPO₄^2-]/[H₂PO₄^-]

log [HPO₄^2-]/[H₂PO₄^-] = -0.20

[HPO₄^2-]/[H₂PO₄^-] = 0.631 and [HPO₄^2-] = 0.631[H₂PO₄^-]

[HPO₄^2-] + [H₂PO₄^-] = 0.1 M

1.631 [H₂PO₄^-] = 0.1 M

[H₂PO₄^-] = 0.0613 M

[HPO₄^2-] = 0.0387 M

For 100 ml (0.1 L), the masses would be:

NaH₂PO₄ = 0.0613 mol/L x 0.1 L x 258 g/mol = 1.58 g

Na₂HPO₄^2- = 0.0387 mol/L x 0.1 L x 143 g/mol = 0.553 g