Question

Concentrated NaOH solution is added to acidic 0.75 M Cd(NO₃)₂. At what pH does the hydroxide compound of Cd²⁺ begin to precipitate? (Assume volume changes are insignificant.) Report your answer to two decimal places. A) pH 2.16 B) pH 5.80 C) pH 7.00 D) pH 10.25

Answer 1

Precipitation of Cd(OH)2 begins at pH 7.00. Option C is answer.

Step-by-step explanation:

The precipitation of Cd(OH)2 occurs when the product of the concentrations of Cd²⁺ and OH⁻ ions in solution exceeds the solubility product constant (Ksp) of Cd(OH)2. The Ksp of Cd(OH)2 is 5.0 × 10⁻¹⁴.

To determine the pH at which precipitation begins, we can set up an expression for the solubility product and solve for the concentration of OH⁻:

Ksp = [Cd²⁺][OH⁻]²

Since the initial concentration of Cd²⁺ is 0.75 M, we can substitute this value into the expression:

5.0 × 10⁻¹⁴ = 0.75[OH⁻]²

Solving for [OH⁻]², we get:

[OH⁻]² = 6.67 × 10⁻¹⁴

Taking the square root of both sides, we find the concentration of OH⁻:

[OH⁻] = 8.17 × 10⁻⁷ M

Now that we know the concentration of OH⁻, we can calculate the pH using the pOH formula:

pH = 14 - pOH

where pOH is the negative logarithm of the hydroxide ion concentration:

pOH = -log10[OH⁻] = -log10(8.17 × 10⁻⁷) = 6.98

Therefore, the initial pH is 14 - 6.98 = 7.02.

Since the initial pH is already close to the pH at which precipitation begins, we can assume that precipitation will begin immediately upon the addition of NaOH. Therefore, the pH at which precipitation begins is 7.00.

Option C is answer.