1. Calcium phosphate has a Ksp of 2.07x10^-33. Determine: a)The molar solubility

Question

1. Calcium phosphate has a Ksp of 2.07x10^-33. Determine:

a)The molar solubility of calcium phosphate.

b)The molar concentration of calcium ions at equilibrium.

c)The molar concentration of phosphate ions at equilibrium.

2. The Ksp of magnesium hydroxide is 2.06x10^-13. You may disregard the normal hydroxide concentration in water

a)What is the molar solubility of magnesium hydroxide?

b) Based on your solubility what is the pH of the resulting solution.

c) Without doing any additional calculations, will magnesium hydroxide be more or less soluble if it is in a solution that is buffer to pH 7.5. Explain your response.

Explanation / Answer

1)

a)Ksp = (3s)^3 * (2s)^2 = 108 s^5

Ksp = 108 s^5

s is molar solubility of calcium phosphate.

so, 108 s^5 = 2.07x10^-33.

solving, s= 113896.22 *10^-12 = 11.389*10^-8 M (moles/litre)

b)The molar concentration of calcium ions at equilibrium. = 3s = 3*11.389*10^-8 = 34.167*10^-8 M.

c)The molar concentration of phosphate ions at equilibrium. = 2s = 2*11.389*10^-8 = 22.778*10^-8 M.

2)

a)Ksp = s*(2s)^2 = 4s^3

where s is the molar solubility of magnesium hydroxide.

4s^3 = 2.06x10^-13

s= 37.205 *10^-6 M.

molar solubility of magnesium hydroxide. = 37.205 *10^-6 M.

b)molar concentration ofhydroxide ions at equilibrium.= 2s = 2* 37.205 *10^-6 M = 74.41*10^-6 M

so, ph = 14- log[OH-] = 14+ log(74.41*10^-6 ) = 9.8716

ph = 9.8716

c)for buffer ph = 7.5 , it is less basic than ph of 9.8716

so, magnesium hydroxide be more soluble to increase the concentration of OH- ions and increase the ph to 9.8716.

,more soluble is the answer.

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