The Effect of a Common Ion on Molar Solubility: pH and OH as a Common Ion If the

ID: 1033777 • Letter: T

Question

The Effect of a Common Ion on Molar Solubility: pH and OH as a Common Ion If the pH of a solution of magnesium chloride gets too high, magnesium hydroxide (insoluble metal hydroxide) could precipitate from solution. If you prepare a buffer solution, it is possible to prevent the precipitation of Magnesium Hydroxide by controling the pH. Which of the following buffers solutions would not result in the precipitation of Magnesium Hydroxide from a solution that is 0.0173 M in Magnesium chloride . Check all that apply Ksp for Mg(OH)2 = 1.2 X 10-11 Weak aci C6H5OH HNO2 HCN 1.3 X 10-10 4.5 X 10-4 4.0 X 10-10 1.0 M C6H5OH with 1.0 M KC6H5O 1.0 M HNO2 with 1.0 M NaNO2 5.6 M C6H5OH with 1.0 M KC6H5O 1.0 M HCN with 2.0 M NaCN 2.0 M C6H5OH with 1.32 M KC6H5O 2.0 M HCN with 1.0 M NaCN

Explanation / Answer

We have the Ksp for Mg(OH)2 as 1.2*10-11 and the solution is 0.0173 M in MgCl2 (this is also the concentration of Mg2+ ions in solution) since MgCl2 ionizes completely in solution to give Mg2+ and Cl- ions).

Ksp = [Mg2+][OH-]2

=====> 1.2*10-11 = (0.0173)*[OH-]2

=====> [OH-]2 = (1.2*10-11)/(0.0173) = 6.9364*10-4

=====> [OH-] = 2.6337*10-5

The [OH-] concentration in the solution is 2.6337*10-5 M. Since we know [OH-], we can obtain pOH as pOH = -log [OH-] = -log (2.6337*10-5 M) = 4.5794.

We define pH + pOH = 14; therefore, pH = 14 – pOH = 14 – 4.5794 = 9.4206 ? 9.42

The pKa of the weak acid must be close to the pH of the solution for the buffer to resist the change in pH. Determine the pKa of the acids using the relation pKa = -log Ka.

Weak Acid

pKa = -log Ka

C6H5OH

1.3*10-10

-log (1.3*10-10) = 9.87

HNO2

4.5*10-4

-log (4.5*10-4) = 3.34

HCN

4.0*10-10

-log (4.0*10-10) = 9.34

The pKa of HCN and C6H5OH are closer to the pH of the Mg(OH)2 solution. Determine the pH of the given combinations.

1.0 M C6H5OH with 1.0 M KC6H5O