In: Chemistry
Consider the titration of a 24.0ml sample of 0.105 M CH3COOH with .130 M Naoh. The La of acetic acid is 1.70x10^-5. what is the a)initial ph? b) the ph at 6.00 of added base? c) the ph at the equivalence point?
Ka = 1.70 x 10-5
pKa = - log Ka = - log [1.70 x 10-5]
pKa = 4.77
millimoles of CH3COOH = 24 x 0.105 = 2.52
a) initially only CH3COOH present so
pH = 1/2 [pKa - logC]
pH = 1/2 [4.77 - log 0.105]
pH = 2.87
b) millimoles of NaOH added = 6.0 x 0.130 = 0.78
2.52 - 0.78 = 1.74 millimoles acid left
0.78 millimoles salt formed
total volume = 24 + 6 = 30 mL
[acid] = 1.74 / 30 = 0.058M
[salt] = 0.78/30 = 0.026M
mixture will act as acidic buffer
pH = pKa + log [salt] / [acid]
pH = 4.77 + log [0.026] / [0.058]
pH = 4.72
c) at equivalence all acid must be convert to salt.
2.52 = V x 0.13
V = 19.38 mL NaOH required
total volume = 24 + 19.38 = 43.38 mL
[salt] = 2.52 / 43.38 = 0.0581 M
pH = 1/2 [pKw + pKa + log C]
pH = 1/2 [14 + 4.77 + log 0.0581]
pH = 8.77