A saturated Mg(OH)2 solution with a volume of 100ml is titrated with 0.50M HCl at 298K. 66.0 microliters of titrant are needed to reach equivalence. How many mols of OH- are present in 100 ml of Mg(OH

Question

A saturated magnesium hydroxide solution with a volume of 100 ml is titrated with 0.50 M HCl at room temperature. 66.0 microliters of titrant are required to reach equivalence. How many moles of OH- are present in 100 ml of saturated magnesium hydroxide?

at equivilance the number of mols should be equal so mols of Mg(OH)2 should also the same as HCl which would be 3.3x10^-5 so wouldn't the number of mols of OH- be 2x(3.3x10^-5) because for every mol of Mg(OH)2 there are 2 of OH. This would equal 6.6x10^-5 which isn't an option? Just wondering where I'm going wrong?

A) 3.3x10^-2 mol

B)1.32x10^-1 mol

C) 3.3x10^-5 mol

D)1.32x10^-4 mol

Answer 1

So, it appears that you know pretty much what you're doing but maybe you didn't have the correctly balanced equation (see below). In my approach, I use umoles (10^-6 moles), and then convert back to moles at the end, but it all comes out the same in the end.

You are correct that there are 3.3x10^-5 moles of HCl, but it takes TWO moles of HCl for each mole of Mg(OH)₂. So moles of Mg(OH)₂ would be 1/2 x 3.3x10^-5 moles. But then there are TWO mols of OH^- per mole of Mg(OH)₂, so you are back to 3.3x10_-5 moles of OH^-.

First, write the balanced equation for the reaction taking place:

Mg(OH)₂ + 2HCl ==> MgCl₂ + 2H₂O .. balanced equation

Next, calculate the micromoles (umols) of HCl used:

0.5 M = 0.5 mols / L = 0.5 umols / ul

66.0 ul x 0.5 umol / ul = 33.0 umols HCl used

Next, using the mole ratio of the balanced equation, calculate umoles Mg(OH)₂ present:

33.0 umols HCl x 1 umol Mg(OH)₂ / 2 umols HCl = 16.5 umols Mg(OH)₂

Finally, convert to moles of OH^-:

16.5 umols Mg(OH)₂ x 2 umols OH^- / umol Mg(OH)₂ = 33.0 umols OH^-

33.0 umols OH^- x 1x10^-6 mols / umol = 3.30x10^-5 moles OH^- ANSWER C