1.

What volume of `0.1M` sodium formate solution should be added to 50 mL of `0.05` M formic acid to produce a buffer solution of `pH= 4.0`? `(pK_(a)` of fomic acid `= 3.80)`

Answer» 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`


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