Question

8. Solid Mg(OH)2 dissociates in water as shown below. The chemical equation for the autoprotolysis of water is also shown. M8OH (aq) + OH"(aq) Mg(OH)2(s) MgOH (aq) Mg2*(aq) + OH (aq) OR Mg(OH)2(s) Mg2*(aq) +20H (aq) H2O*(aq) + OH (aq) 2H20( The middle equation can be represented in either of the two ways shown since the product ions are the same in both cases. For this solution, (A) write the correct charge-balance equation. [5 points] (B) write the correct mass-balance equation for [OH-]. [10 points] 9. Rate the difficulty of this exam on an integer scale from 1 to 10, with 1 being ridiculously easy and 10 being outrageously difficult. [1 point for any answer; 0 points for no answer

Answer 1

A) charge balance equation =

2[Mg2+] + H3O+ = [OH-]

B) mass balance equation of OH-

Let amount of hydroxide in solution = X

Now in the solution, hydroxide ion comes from Mg(OH)2 as well as from water.

Now suppse y amount of OH- comes from Mg(OH)2

Hence y = 2M

Where M is the amount of Mg2+

Now Z amount of OH- comes from water which is equal to amount of H+

Which means Z = [ H+]

Now from above steps we know that

X=y+Z

X = 2M + [H+]

From the question given, we can say that in the solution magnesium exist in two forms - [Mg2+] and [MgOH+]

So M = [Mg2+] + [MgOH+]

Now

X = 2[Mg2+] + 2[MgOH+] + [H+]

For balancing in terms of OH-, Mg2+ can be ignored

So mass balance equation of OH-

= X = 2[MgOH+] + [H+]