What volume (inml) of `0.10 M` sodium formate solution should be added to `50 ml` of `0.05M` formic acid to produce a buffer solution of `pH4. [pK_(a)` for formic acid is 3.7]

Explanation

Let V mL of `0.1M HCOONa` be mixed to 50 mL of `0.05 M HCOOH`.
`[.: [ ] = ("Total millimole")/("Total volume")]`
`:. "In mixture" [HCOONa]= (0.1xxV)/((V+50))`
`[HCOOH]= (50xx0.05)/(V+50)`
`:. pH= -log K_(a)+log ((["Salt"])/(["Acid"]))`
`:. 4.0=3.80+log (((0.1xxV)//(V+50))/(2.5//(V+50)))`
`:. V = 39.62 mL`