Question

Consider a buffer solution that contains 0.25 M HCO2H and 0.20 M HCO2Na. pKa(HCO2H)=3.75.

1. Calculate its pH

2. Calculate the change in pH if 0.135 g of solid NaOH is added to 180 mL of this solution.

3. If the acceptable buffer range of the solution is ±0.10 pH units, calculate how many moles of H3O+ can be neutralized by 250 mL of the initial buffer.

Answer 1

a) We will use Hendersen's equation here to calcualte pH of buffer

pH = pKa + log [Salt] / [acid]

pKa = 3.75

[Salt] = 0.2 M

[Acid] = 0.25 M

pH = 3.75 + log [0.2 / 0.25] = 3.65

b) If we will add NaOH to the buffer then the base will react with acid to form salt

The moles of salt formed = Moles of NaOH added = Moles of acid reacted

Moles of NaOH added = Mass / Molecular weight = 0.135/40 = 0.003375 moles

Moles of acid present initially = Molarity X volume (L) = 0.25 X 0.180 = 0.045

Moles of acid left = 0.045 - 0.003375 = 0.0416 moles

Moles of salt present = 0.2 X 0.180 = 0.036 moles

Moles of salt after addition of NaOH = 0.036 + 0.003375 = 0.0394 moles

New pH will be

pH = pKa + log[salt ] / [Acid] = 3.75 + log [0.0394/ 0.0416] = 3.73