Question

30 CC of M/4 HCl, 20 CC of M/2 HNO₃, and 40 CC of M/4 NaOH solutions are mixed and the volume is made upto 1 dm³. The pH of the resulting solution is:

(a) 2
(b) 1
(c) 3
(d) 8

Answer 1

Correct option is (a) 2

Total millimoles of H^–

= (30 x \(\frac 13\)) + (20 x \(\frac 12\))

= 10 + 10 = 20

Total millimol of OH^–

= 40 x \(\frac 14\) = 10

∴ H⁺ ions left after the neutralisation = 10 millimoles Volume of the solution

= 1 dm³

∴ Molarity of H⁺ ions

= \(\frac{10}{1000}\) = 10^-2M

pH = – log [H⁺]

= – log [10^-2] = 2