Question

what is the pH when a 5 mL solution of 30 mL HC2H3O2 is diluted with 25 mL of H2O? The pH of 30 mL HC2H3O2 solution is 2.52 & pKa= 4.74

Answer 1

Stock solution is 30mL acid with pH = 2.52 and pKa = 4.74

The concentration of acid to be calculated from the formula

pH = 1/2 [ pKa -log C]

Now 5 mL of this solution is diluted with water to make it 25 mL.

The [acid] / molarity is calculated as M1V1 = M2V2 .

i) concentration of 30mL solution.

2.52 = 1/2[4.74 -log C]

log C = -0.30

and C= 0.5M

Part 2 )5 mL of this 0.5M solution is diluted to 25 mL

5x0.5 = 25 x M

or mmolarity = 0.1 M

pH of this solution = 1/2 [4.74 - log 0.1]

= 1.87