Question

A buffer is prepared by mixing 25.0 ml of 1.00 NaC2H3O2 with 25.0 ml of 0.900 M HC2H3O. If 20 ml of 0.50 M HCl is added to the initial buffer solution, what is the pH?

Answer 1

by adding HCl to the buffer following reaction takes place

C2H3O2^- + H⁺   ---------------------> HC2H3O2

millimoles of NaC2H3O2 = 25 x 1.0 = 25

millimoles of HC2H3O2 = 25 x 0.9 = 22.5

millimoles of HCl added = 20 x 0.5 = 10

after HCl added

millimoles of NaC2H3O2 = 25 - 10 = 15

millimoles of C2H3O2 = 22.5 + 10 = 32.5

total volume = 25 + 25 + 20 = 70

[NaC2H3O2] = 15 / 70 = 0.214 M

[HC2H3O2] = 32.5 / 70 = 0.464 M

pKa of HC2H3O2 = 4.75 standard value

pH = pKa + log [NaC2H3O2] / [HC2H3O2]

pH = 4.75 + log [0.214] / [0.464]

pH = 4.75 - 0.336

pH = 4.414