Calculate the volume of 0.1 M acetic acid solution to be mixed with `50cm^(3)` of 0.2M sodium acetate solution, in order to prepare a standard buffer of pH 4.94 (`pK_(a)` of acetic acid = 4.74).

Explanation

`pH=pK_(a)+"log"(["salt"])/(["acid"])`
`4.94=4.74+"log"(["salt"])/(["acid"])`
`"log"(["salt"])/(["acid"])=4.94-4.74=0.20`
`(["salt"])/(["acid"])="anti"log0.20=1.58`
That is, the ratio of concentrations of salt : acid is 1.58 : 1
Let V be volume of 0.1 M acetic acid to be mixed
`therefore(["salt"])/(["acid"])=((50xx2)/(1000))/((Vxx0.1)/(1000))=(1.58)/(1)`
`therefore` Volume of acetic acid, `V=(50xx0.2xx1)/(0.1xx1.58)=90.4cm^(3)`.