The value of Ksp for Mg₃₁(AsO₄)₂₂ is 2.1 * 10⁻²⁰. The AsO₄³⁻ ion is derived from the weak acid H₃AsO₄ with pKa₁ = 2.22, pKa₂ = 6.98, pKa₃ = 11.502. (b) Calculate the pH of a saturated solution of Mg₃₁(AsO₄)₂₂ in water.

Name: Chemistry: The Central Science
Author: Brown
ISBN: 9780134414232

Verified step by step guidance

Identify the dissolution reaction for Mg_{31}(AsO_4)_{22} in water: Mg_{31}(AsO_4)_{22} \(\rightleftharpoons\) 31Mg^{2+} + 22AsO_4^{3-}.

Write the expression for the solubility product constant (K_{sp}): K_{sp} = [Mg^{2+}]^{31} [AsO_4^{3-}]^{22}.

Assume the molar solubility of Mg_{31}(AsO_4)_{22} is 's'. Then, [Mg^{2+}] = 31s and [AsO_4^{3-}] = 22s.

Substitute the concentrations into the K_{sp} expression: K_{sp} = (31s)^{31} (22s)^{22} = 2.1 \(\times\) 10^{-20}.

Use the concentration of AsO_4^{3-} to find the pH. Since AsO_4^{3-} is derived from H_3AsO_4, consider the hydrolysis reaction: AsO_4^{3-} + H_2O \(\rightleftharpoons\) HAsO_4^{2-} + OH^{-}. Calculate the pH using the equilibrium expression and the known pK_a values.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is an equilibrium constant that quantifies the solubility of a sparingly soluble ionic compound. It is defined as the product of the molar concentrations of the ions, each raised to the power of their coefficients in the balanced dissolution equation. For Mg₃₁(AsO₄)₂₂, Ksp helps determine how much of the compound can dissolve in water, influencing the concentration of the AsO₄³⁻ ions in solution.

Acid-Base Equilibrium and pKa

The pKa value is a measure of the strength of an acid, indicating the pH at which half of the acid is dissociated. In this case, H₃AsO₄ has three pKa values, reflecting its ability to donate protons in a stepwise manner. Understanding these pKa values is crucial for determining the predominant species of arsenate ions in solution, which affects the pH and the overall chemistry of the saturated solution.

pH Calculation in Saturated Solutions

The pH of a saturated solution can be calculated by considering the concentration of hydrogen ions produced from the dissociation of the weak acid formed by the dissolved salt. In this scenario, the concentration of AsO₄³⁻ ions will influence the equilibrium of the acid-base reaction, allowing for the calculation of the pH based on the concentration of these ions and their corresponding dissociation constants.

Recommended video:

Guided course

02:15

pH Calculation Example

Related Practice

Textbook Question

The solubility product for Zn1OH22 is 3.0 * 10-16. The formation constant for the hydroxo complex, Zn1OH242 - , is 4.6 * 1017. What concentration of OH- is required to dissolve 0.015 mol of Zn1OH22 in a liter of solution?

Textbook Question

The value of K_sp for Cd(OH)₂ is 2.5 × 10^-14. (a) What is the molar solubility of Cd(OH)₂?

Textbook Question

A solid sample of Fe1OH23 is added to 0.500 L of 0.250 Maqueous H2SO4. The solution that remains is still acidic. Itis then titrated with 0.500 M NaOH solution, and it takes12.5 mL of the NaOH solution to reach the equivalencepoint. What mass of Fe1OH23 was added to the H2SO4solution?

Textbook Question

The solubility product constants of PbSO4 and SrSO4 are 6.3 * 10-7 and 3.2 * 10-7, respectively. What are the values of 3SO4 2 - 4, 3Pb2 + 4, and 3Sr2 + 4 in a solution at equilibrium with both substances?

Textbook Question

The value of K_sp for Cd(OH)₂ is 2.5 × 10^–14. (b) The solubility of Cd(OH)₂ can be increased through formation of the complex ion CdBr₄^2- (K_f = 5 × 10³). If solid Cd(OH)₂ is added to a NaBr solution, what is the initial concentration of NaBr needed to increase the molar solubility of Cd(OH)₂ to 1.0 × 10^-3 mol/L?