Question

A 280.0 −mL buffer solution initially contains 2.5×10−2 M of HCHO2 and 2.5×10−2 M of NaCHO2.

a,) In order to adjust the buffer pH to 4.10, should you add NaOH or HCl to the buffer mixture?

The answer is: NaOH

b.) What mass of the correct reagent should you add?

Answer 1

mass of the NaOH reagent should you add to maintain pH - 4.10

If a solution contains equal molar concentrations of both the acid HCHO2 and the salt NaCHO2, it would have a pH of 3.74

how----?, lets see

The K_a for formic acid is 1.8 x 10^-4.

Ka = x * [H⁺] [HCOO^-]/ HCOOH =

1.8 x 10^-4 = x = [H⁺]

pH = -log [H⁺] = 3.74

Buffer: pH = 3.74

now, adding y M of NaOH

The concentration of HCOOH would change from 2.5×10−2 M to 2.5×10−2 M - y M and the concentration of HCOO^- would change from 2.5×10−2 M to2.5×10−2 + y M

pH is 4.10

Ka = -log[4.10] * 2.5×10−2 + y / 2.5×10−2 M - y = 1.8 x 10^-4

solving for y

y = 9.6X10-3 M

in 280mL buffer solution NaOH to be added = 0.1079g