Answer to Question #177078 in Chemistry for dafd

Question

1) In a saturated solution of Zn(OH)₂ at 25°C, the value of [OH^-] is 2.0 x 10^-4 M. What is the value of the solubility-product constant, Ksp, for Zn(OH)₂ at 25°C ? Explain

A)4.0 x 10, -18

B)8.0 x 10, -12

C)4.0 x 10^-12

D)2.0 x 10^-4

2) CaF₂(s)⇄Ca²⁺(aq)+2F⁻(aq)          Ksp=4.0×10⁻¹⁰

The concentration of F⁻(aq) in drinking water that is considered to be ideal for promoting dental health is 4.0×10⁻⁵M. Based on the information above, the maximum concentration of Ca²⁺(aq) that can be present in drinking water without lowering the concentration of F⁻(aq) below the ideal level is closest to

• 0.025 M
• 1.6 x 10^-15 M
• 1.6 x 10^-6 M
• 0.25 M

*EXPLAIN*

Answer 1

Solution:

(1):

Zn(OH)₂(s) ⇄ Zn²⁺(aq) + 2OH⁻(aq)

___x_______ ___x__________2x_____

Hence,

[OH⁻] = 2x = 2.0×10^-4 M

[Zn²⁺] = x = [OH⁻] / 2 = (2.0×10^-4 M) / 2 = 1.0×10^-4 M

The K_sp expression for zinc hydroxide is:

K_sp = [Zn²⁺][OH⁻]²

K_sp = (1.0×10^-4) × (2.0×10^-4)² = 4×10^-12

K_sp = 4×10^-12

NB!: K_sp for Zn(OH)₂ is 5.0×10^-17 (Table value. Maybe there is a mistake in the problem statement).

Answer (1): C) 4×10^-12

(2):

CaF₂(s) ⇄ Ca²⁺(aq) + 2F⁻(aq)

The K_sp expression for calcium fluoride is:

K_sp = [Ca²⁺][F⁻]²

K_sp= 4.0×10⁻¹⁰

[F⁻] = 2x = 4.0×10⁻⁵ M

Hence,

[Ca²⁺]_max = K_sp / [F⁻]²

[Ca²⁺]_max = (4.0×10⁻¹⁰) / (4.0×10⁻⁵)² = 0.25

[Ca²⁺]_max = 0.25 M

Answer (2): [Ca²⁺]_max = 0.25 M.