Question

Calculate the concentration of the parent acid, H₂A, in the sample given that 10.00 mL aliquot of the acid solution was analyzed after adding 25.00 mL of water to allow proper use of the pH electrode. (Ka₁ =0.079433)

Answer 1

In glass electrode or ph electrode pH is maintained at 7. [H+] = 1×10⁻⁷M

Let the concentration of the acid after the dilution = M

H2A <==> H+ + HA^-

	H2A	H+	HA^-
initial	M	0	0
change	-x	+2x	+x
equilibrium	M-x	2x	x

x = 1*10^-7

x = 0.5 *10^-7 M

Ka = x^2/(M-x)

0.079433 = (0.5 *10^-7) ^2/(M-0.5 *10^-7)

M = 5.0 *10^-8

Concentration of the parent acid (before dilution ) can be calculated as follows.

M1V1 = m2V2 where M1 and V1 is the concentration and volume of parent acid.M2 and V2 is the concentration and volume of diluted acid.

M1 = M2V2 /V1 = 5.0 *10^-8 * 35/10 = 1.75*10^-7 M