Question

QUESTION 1:

A buffer is prepared by mixing 192 mL of 0.452 M HCl and 0.485 L of 0.400 M sodium acetate. (See this table.) pH is 4.83.

How many grams of KOH must be added to 0.500 L of the buffer to change the pH by 0.18 units?

QUESTION 2:

60.0 mL of a 0.350 M solution of a weak acid are titrated with 0.250 M NaOH. What is the pH when 57% of the acid has been titrated? (Ka = 8.87 ✕ 10-8)

a)3.75

b)7.17

c)6.93

d)7.05

Answer 1

No of mol of HCl added = 0.192*0.452 = 0.0868 mol

No of mol of sodium acetate = 0.485*0.4 = 0.194 mol

pH = pkA + log(salt/acid)

4.83 = x + log((0.194-0.0868)/0.0868)

x = pka = 4.74

addition of KOH increases the PH

so that final pH = 4.83+0.18 = 5.01

5.01 = 4.74 + log((0.1072+x)/(0.0868-x))

x = 0.019 mol

mass of KOH added = 0.019*56 = 1.064 grams

q.No2:

No of mol of acid = 60/1000*0.35 = 0.021 mol

No of mol of NaOH added = 0.021*57/100 = 0.012 mol

pH = pka+log(salt/acid)

pka = -log(8.87*10^-8) = 7.05

    = 7.05+log(0.012/(0.021-0.012))

    = 7.17

answer: b) 7.17