Question

You have to prepare a pH 5.00 buffer, and you have the following 0.10M solutions available: HCOOH, HCOONa, CH3COOH, CH3COONa, HCN, and NaCN.

Part A

Which solutions would you use?

Check all that apply.

	HCOOH
	HCOONa
	CH3COOH
	CH3COONa
	HCN NaCN
	Part B How many milliliters of each solution would you use to make approximately a liter of the buffer?

Answer 1

The pH of the buffer is given by the Henderson - Hasselbalch equation:
pH = pKa + log ([salt]/[acid])

pKa for CH3COOH = 4.74

So we can use a combination of CH3COOH and CH3COONa

Substitute in the equation

pH = pKa + log ([salt]/[acid])

5.00 = 4.74+ log ([salt]/[acid])

so log ([salt]/[acid]) = 0.26
[salt]/[acid] = 10^0.26
[salt]/[acid] = 1.819 = 1.82

The molar ratio of salt to acid must be 1.82

so we must take 1.82 moles of CH3COONa along with 1.0 mole of CH3COOH

Because the molar concentrations of the two components are equal , and we need a final volume of 1000mL
Volume of CH3COONa = 1.82/(1 + 1.82) *1000 = 645.3 mL
Volume of CH3COOH = 1.0/(1 + 1.82) *1000 = 354.6 mL