In: Chemistry
Calculate the pH for each of the following cases in the titration of 50.0mL of 0.160M HClO(aq) with 0.160M KOH(aq).
a) before addition of any KOH
b) after addition of 25.0 mL of KOH
c) after addition of 40.0 mL of KOH
d) after addition of 50.0 mL of KOH
e) after addition of 60.0 mL of KOH
pKa = -logKa = = -log(4.0 x10^ -8.) = 7.4
millimoles of HClO= 50 x0.160= 8
a) 0 ml KOH added
pH = 1/2 (pKa- log C)
= 1/2 (7.4 -log (0.160) ) = 4.10
pH= 4.10
(b) after addition of 25.0 mL of KOH
it is first equivalece point here pH = pKa
pH = 7.4
(c) after addition of 40.0 mL of KOH
millimoles of KOH = 40 x 0.160 = 6.4
HClO + KOH ------------------------------> KClO + H2O
8 6.4 0 0 -----------------------initial
1.6 0 6.4 6.4-------------------equilibirum
in the solution acid and salt remained so it can form buffer
For acidic buffer
Henderson-Hasselbalch equation
pH = pKa + log[salt/acid]
= 7.4 + log (6.4/1.6)
= 8.00
pH = 8.00
(d) after addition of 50.0 mL of KOH
millimoles of KOH = 0.160 x 50 = 8
HClO + KOH ------------------------------> KClO + H2O
8 8 0 0 -----------------------initial
0 0 8 8----------------equilibirum
in the solution salt remained so we have to use salt hydrolysis.
it is the salt of strong base and weak acid so pH should be more than 7
[salt] = salt millimoles /total volume in ml
= 8 /(50+50)
= 0.08 M
pH = 7 + 1/2[Pka + logC]
= 7 + 1/2 [7.4 + log (0.08)]
= 10.15
pH = 10.15
e) after addition of 60.0 mL of KOH
millimoles of KOH = 0.160 x 60 = 9.6
HClO + KOH ------------------------------> KClO + H2O
8 9.6 0 0 -----------------------initial
0 1.6 8 8-----------------equilibirum
in the solution strong base remained
[base ] = 1.6 /total volume = 1.6 /110 = 0.0145
pOH = -log[OH-] = -log(0.0145) =1.84
pH + pOH = 14
pH = 12.16