0.1M weak acid HA (pH = 3) is titrated with .05M NaOH soln. Calculate the pH when 25% of acid solution has been neutralised.
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first of all, we have to find of given acid.
so,
here, pH = 3
from formula ,
so, 3 = - log[H3O^+]
or, [H3O^+] = 10^-3 mol/L
at equilibrium,
[H3O^+] = [A^-] = 10^-3 mol/L
and [HA] = initial [HA] - [H3O+]
= 0.1 - 10^-3
= 0.1 - 0.001
= 0.099 mol/L
so,
Ka = 1.01 × 10^-5
and pKa = -log(1.01 ×10^-5) ≈ 5
Suppose we have taken 10 ml of 0.1M weak acid, HA . For complete neutralisation 20 ml of 0.05 NaOH will be required.
so for 25% neutralisation 5ml of NaOH will be required.
at this point,
number of millimole of HA present in the titration mixture = 0.75 millimole
The number of m.moles of salt present in the mixture = 0.25 millimole.
now,
= 5 + log(0.25/0.75)
= 5 + log(1/3)
= 5 - log3
= 4.522
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