chemistry problems - Calculating hydrogen ion concentration?

J

Jason P

New member
Calculate the hydrogen ion concentration and hence the pH of the following solutions of weak acids:

a) 0.10 mol of ethanoic acid: pKa: 4.8
b) 0.10 mol of propnoic acid: pK 4.9
c) 0.15 mol of chloric(I) acid: pKa 7.4


If you don't want to tell me the answer - just give me the calculation on how to work it out. Or if both.

Thanks for your time.

Jason -
 
Consider a general weak acid (monobasic)

HA --> H+ + A-
t=0 a 0 0
t=teq a(1-x) ax ax

Ka = [H+][A-] = a^2x^2 = (ax)^2

=> ax = Ka^0.5

but ax = [H+]

=> [H+] = Ka^0.5

=> pH = -log[H+] = -log(Ka)^0.5 = -0.5logKa

Now you can put the values as given
 
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