Phosphoric acid is a diprotic acid, and a 0.20 M, 50.0ml sample is titrated with
ID: 1387254 • Letter: P
Question
Phosphoric acid is a diprotic acid, and a 0.20 M, 50.0ml sample is titrated with 12.5 ml of 0.100M potassium hydroxide to reach the first equivalent point, where the first acid dissociation constant Ka1= 7.25x10-3M at 25 degrees celcius. an additional 12.5 mL, 0.100M potassium hydroxide is added to reach the first equivalence point, where the second acid dissociation constant is Ka1= 6.31x10-8M at 25 degrees celcius, and the acid is neutralized.
1.) Calculate the pH for the first equivalence point
2.) Calculate the pH for the second equivalence point
Explanation / Answer
1)
Phosphoric acid is a diprotic acid
mole=molarity*volume
=0.2*0.05
=0.01
mole KOH=0.1*0.0125
=0.0125
after this titration 0.0025mole 0.0125HSO3-
Ka1= 7.25x10-3M
H3O+=7.25*10-3*0.0025/0.0125
=1.45*10-3
PH=-log(1.45*10-3)
=2.838
2)
Ka1= 6.31x10-8M
H3O+=6.31*10-8*0.0025/0.0125
=1.262*10-8
ph=-log(1.262*10-8)
=7.898