29 11. (15 points) Cr(OH)3 (s) has a Ksp of 3.0x 10 A. Write a reaction represen

Question

29 11. (15 points) Cr(OH)3 (s) has a Ksp of 3.0x 10 A. Write a reaction representing the dissolution of Cr(OH)3 (). B. Write the Ksp equilibrium constant expression. C. Will a saturated aqueous solution of Cr(OH)s (s) be acidic, neutral, or basic? D. Calculate |C3) in a saturated aqueous solution of C r(OH)s (s) prepared in pure water. E. Calculate [Cr"] in a saturated aqueous solution of Cr(OH)3 (s) prepared in a solution buffered to pH = 8.00. You may assume that the buffer maintains this pH even after dissolution of small amounts of the solid. Hint: this is a common ion effect situation - the buffer fixes the hydroxide concentration at t value corresponding to pH-8.60, and you should use this (OH ] in the Ksp expressio solving for ICr 1. 3+

Explanation / Answer

a)

the reaction shos how solid will form aqueous solution

Cr(OH)3(s) <--> Cr3+(aq) + 3OH-(aq)

b)

Ksp data is given by

Ksp = [Cr3+][OH-]^3

c)

if

Cr(OH)3 is saturated, then

Ksp = [Cr+3][OH-]^3

3*10^-29 = S*(3S)^3

27*S^4 = 3*10^-29

S = (3*10^-29) / (27) )^(1/4)

S = 3.246*10^-8

[OH-] = 3*3.246*10^-8 = 9.738*10^-8

pOH = -log( 9.738*10^-8) = 7.01

this is almost neutral

d)

from previous data

[Cr+3] = S =  3.246*10^-8 M

e)

if pH = 8, then pOH = 14-8 = 6

[OH-] = 10^-6

Ksp = [Cr+3][OH-}^3

3*10^-29 = [Cr+3]*(10^-6)^3

[Cr+3] = (3*10^-29) / ((10^-6)^3) = 3*10^-11 M

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