Question

Calculate the ph of a 1.00 liter buffer solution that contains 0.300 m hc2h3o2 and 0.300 m nac2h3o2. (b) calculate the ph of this solution after 0.020 mol of naoh is added (neglect any volume changes

Answer 1

(a) 4.75; (b) 4.81

(a) pH of buffer

Step 1. Write the equation for the equilibrium

HC2H3O2 + H2O ⇌ C2H3O2^(-) + H3O^(+); pKa = 4.75

or

HA + H2O ⇌ A^(-) + H3O^(+)

Step 2. Calculate the pH of the buffer

The Henderson-Hasselbalch equation is

pH = pKa + log([A^(-)]/[HA]) .

pH = 4.75 + log((0.300 mol/L)/(0.300 mol/L)) = 4.75 + log1.00 = 4.75 + 0

= 4 .75

(b) pH of buffer after adding NaOH

Step 1. Calculate the new concentrations of HA and A^(-)

We can use an ICE table for the calculation.

_______HA + H2O ⇌ H3O^(+) + A^(-)

I/mol:__0.300_______0.300

C/mol:_-0.020______+0.020

E/mol:__0.280_______0.320

pH = pKa + log([A^(-)]/[HA]) = 4.75 + log((0.320 mol/L)/(0.280 mol/L))

= 4.75 + log1.143 = 4.75 + 0.058 = 4 .81

This result makes sense because adding a base to a buffer should increase its pH.