Given that Buffer A contains 200 ml of 0.05 M HOCl and 400 ml 0.03 M NaOCl, calc
ID: 894919 • Letter: G
Question
Given that Buffer A contains 200 ml of 0.05 M HOCl and 400 ml 0.03 M NaOCl, calculate the pH of the buffer solution, the pH of the solution after adding 10 mL of 0.5 M HCl, and the pH of the solution after adding 20 mL of 0.4 M NaOH.
Given that Buffer B contains 200 mL 0.5 HOCl and 400 mL 0.3 M NAOCl, calculate the pH of the buffer solution, the pH of the solution after adding 10 mL of 0.5 M HCl, and the pH of the solution after adding 20 mL of 0.4 M NaOH.
Which buffer will resist pH changes better?
Explanation / Answer
pH of acidic buffer
pH = pka + log [salt/acid]
No of moles of HOCl = 200/1000*0.05 = 0.01 mole
No of moles of NaOCl = 400/1000*0.03 = 0.012 mole
pka of HOCl = 7.54
pH = 7.54+log(0.012/0.01) = 7.62
after additing HCl
No of moles of HCl added = (10/1000)*0.5 = 0.005 mole
pH = 7.54+log((0.012-0.005)/(0.01+0.005)) = 7.21
after adding NaOH
No of moles of NaOH added = (20/1000)*0.4 = 0.008 Mole
pH = 7.54+log((0.012+0.008)/(0.01-0.008)) = 8.54
2.
pH of acidic buffer
pH = pka + log [salt/acid]
No of moles of HOCl = 200/1000*0.5 = 0.1 mole
No of moles of NaOCl = 400/1000*0.3 = 0.12 mole
pka of HOCl = 7.54
pH = 7.54+log(0.12/0.1) = 7.62
after additing HCl
No of moles of HCl added = (10/1000)*0.5 = 0.005 mole
pH = 7.54+log((0.12-0.005)/(0.1+0.005)) = 7.58
after adding NaOH
No of moles of NaOH added = (20/1000)*0.4 = 0.008 Mole
pH = 7.54+log((0.12+0.008)/(0.1-0.008)) = 7.68
Buffer solution with higher concentration resists the change in pH better.
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