Question

The volume of `0.2 M NaOH` needed to prepare a buffer of `pH 4.74` with `50 mL` fo `0.2M` sodium dicholoroacetate acid `(pK_(b) ` of `CH_(3)COO^(-)=9.26)` is `:`
A. `50mL`
B. `25mL`
C. `20mL`
D. `10mL`

Answer 1

Correct Answer - 2
Let VmL of NaOH be needed to give `CH_(3)COONa`
`{:(NaOH,+,CH_(3)COOH,rarr,CH_(3)COONa,+,H_(2)O),(0.2xxV,,50xx0.2,,0,,0),(-,,[10-0.2V],,0.2V,,0.2V):}`
`:. " "pH=pK_(a)+log.(["Salt"])/(["Acid"])=pK_(a)-pK_(a)+log.(["Salt"])/([Aci d])=14-9.26+log.(["Salt"])/(["Acid"])`
`=14-9.26+log.([(0.2V)/(50+V)])/([(0-0.2V)/(50+V)])`
`4.74=4.74+log.[(0.2V)/(10-0.2V)]" ":." "V=(10)/(0.4)=25mL`