What volume of 6.0 M HCl would you need to add to a 800.0 mL of 0.10 M NaAc to give a final pH = 4.30? The pKa of HAc is 4.78. Give the answer in milliliters, using 2 sig. figs.

3.33 ml

Explanation:

We have

pH to be equal to pKa + log [ NaAc / HAc]

pH = 4.30

pKA = 4.78

This is expressed as

4.30 = 4.78 + log [ NaAc / HAc]

log [ NaAc / HAc] = - 0.48

[ NaAc / HAc] = 10^-0.48

= 0.33

Naac/HAc = 0.33

From here we cross multiply to get

NaAc = 0.33 HAc

The ratio can be given as 1 : 0.33 or 3:1

Moles of NaAc = 800 x 0.10 / 1000

= 0.08

With a ratio of 3 : 1 NaAC and HAc

We have 0.06 mol NaAc and 0.02 mol HAc

moles of HCl = 0.02 mol

Volume of Hcl = 0.02x1000 = 20

20/6.0

= 3.33 Ml

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