A 250.0-mL buffer solution initially contains 0.025 mol of HCHO2 and 0.025 mol of NaHCO2. In order to adjust the buffer pH to 4.10, should you add NaOH or HCl to the buffer mixture? What mass of the correct reagent should you add?

We need to add NaOH to increase the pH decreasing the H⁺ concentration.

0.4g of NaOH must be added

Explanation:

The pKa of HCHO2 is 3.74. Using H-H equation the initial pH of the buffer is:

pH = pKa + log [NaHCO2] / [HCHO2]

pH = 3.74 + log [0.025mol] / [0.025mol]

pH = 3.74

The initial pH is 3.74, as we want a pH of 4.10, we need to add NaOH to increase the pH decreasing the H⁺ concentration.

Total moles of HCHO2-NaHCO2 buffer are:

0.025mol + 0.025mol =

0.050mol = [HCHO2] + [NaHCO2] (1)

Using H-H equation:

4.10 = 3.74 + log [NaHCO2] / [HCHO2]

0.36 = log [NaHCO2] / [HCHO2]

2.29087 = [NaHCO2] / [HCHO2] (2)

NaOH reacts with HCHO2 as follows:

NaOH + HCHO2 → NaHCO2 + H2O

Replacing (1) in (2):

2.29087 = 0.050 - [HCHO2] / [HCHO2]

2.29087[HCHO2]  = 0.050 - [HCHO2]

3.29087[HCHO2]  = 0.050

[HCHO2] = 0.015 moles

As HCHO2 decreases from 0.025 moles to 0.015 moles, the moles of NaOH added are 0.010moles.

The mass is:

0.010moles NaOH * (40g/mol) =

it is b that is the answer.