Question

1. In your role as a corrosion scientist, you are required to undertake corrosion rate measurements in the laboratory.

(a) For some initial measurements, you have prepared an aqueous solution of MgCl₂. As a next step, you increase the pH of the solution to the desired value. A white precipitate is formed. Explain this observation.

(b) For further measurements, you intend to prepare the following two solutions:

(i) 0.17 g of Ba(OH)₂ in 100 ml of H₂O;

(ii) 0.01M H₂SO₄ (pK_a1 = -2, pK_a2 = 1.92).

Calculate the pH of each of these solutions.

Answer 1

(a)

As the pH is increased, the conc. of OH^- ions increases in the solution, and the Mg²⁺ ion forms the compound Mg(OH)₂, which is insoluble and hence forms a white ppt.

(b)

(i)

Conc. of Ba(OH)₂ solution = (0.17/171.3)/0.1 = 0.00992 M

So, conc. of OH^- ions = 2*0.00992 = 0.01984 M

Thus, pOH = 1.7

So, pH = 14-1.7 = 12.3

(ii)

Since H₂SO₄ is a very strong acid ( having very less pKa values, we assume that it dissociates completely)

So, [H⁺] = 0.01*2 = 0.02

Thus, pH = -log(0.02) = 1.7