Question

12. The pH of a 0.063 M weak monoprotic acid is 3.61. Calculate the Ka of the acid.

Answer 1

Given :Molarity of monoprotic weak acid =0.063M

pH of weak acid =3.61

Weak acid is represented as HA

HA--->H++A-

The concentration of H+ion is not the same as the molarity of weak acid. Thus, actual concentration is calculated from pH.

pH = - Log [H+​​​​​​]

[H+​​​​​​] = 10-pH

[H+​​​​​​] = 10-3.61

= 0.000245

Now construct ICE table

I - INITIAL CONCENTRATION

C - CHANGE IN CONCENTRATION

E- EQUILIBRIUM CONCENTRATION

	HA	H+	A-
I	0.063	0	0
C	-0.000245	0.000245	0.000245
E	0.063-0.000245=0.062755	0.000245	0.000245

Acidity constant is the ratio of the product of concentration of product to the product of the concentration of reactants each term raised to power equal to its stoichiometric coefficient as given in the balanced chemical equation.

Ka = [H+] [A-] /[HA] =0.000245*0.000245/0.062755

=6.0025*10-8/0.062755

=95. 6497*10-8 =9.56497*10-7

After round off, ka of monoprotic acid is 9.565*10-7