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What is final concentration of Pb2+ ion when mixing PbCl2 and H2SO4

Submitted by Sunil on Sat, 05/10/2008 - 16:20

What is the final concentration of lead ions, [Pb2+], in solution when 100 ml of 0.10 M PbCl2 (aq) is mixed with 100 ml of 0.050 M H2SO4 (aq)?

well the first thing you want to check is if PbCl2 is a strong electrolyte

if it is then you just need to figure out how many moles of PbCl2 you have

Molarity x Volume = moles PbCl2

moles of PbCl2 = moles of Pb2+

since the other solution doesn't have a common ion

you will just add up the volume of both solution and divide the moles of Pb2+ by that

PbCl2 is a strong electrolyte, because its ionic compound.

So now we need to find moles of Pb2+ = moles of PbCl2

0.10 M PbCl2 x 0.1 L = 0.01 mol PbCl2 = 0.01 mol Pb2+ = 0.05 mol Cl-

So now, 0.01 mol / 0.2 L = 0.05 M (how do u do this on paper?)

but the choices are:
A) 0.005 M
B) 0.012 M
C) 0.025 M
D) 0.250 M
E) 0.10 M

PbCl2 is not a strong electrolyte

it can be ionic and only semi soluble

you will need to find its Ksp

r u sure that I need to find its Ksp. Because all the information given is what I wrote above.

Cant you just find the moles of both reactants, then find which one is the limiting reactant and from there find out the amount of PbSo4 that will be made. After that I found that you can only make 0.005 moles of PbSO4, now dividing that 0.2 L of volume gives me 0.025 M of Pb2+ ions.

thats right, i was PbSO4 is less soluble than PbCl2 so it will precipitate

you are exactly right

i feel like a dummy now

I think there's some confusion about strong electrolytes.  I learned that ALL salts are strong electrolytes, although some salts are only slightly soluble.  In other words, the portion of any salt that dissolves will be completely dissociated.

PbCl2 is a case in point, it has a Ksp value of 1.2 x 10-5. 

Based on that it is not possible to have a .1M solution of PbCl2  Such a solution would consist of a [Pb+2]=.1M and a [Cl-]=.2M
                        Plugging into the Ksp expression gives a value of
                          [Pb+2][Cl-]^2 = .1 x .2^2 = .004M
Since that exceeds the Ksp value PbCl2 would have to precipitate (thereby lowering the Pb+2 concentration).

However, given that they are providing you with answers, it would be possible to make an educated quess based on the fact that PbSO4 is VERY insoluble (the Ksp value is 1.8 x 10-8).  Give the fairly large concentration of the SO4-2 ion, it is almost certain that the concentration of the Pb+2 would be the smallest value given (.005M).

Because the PbSO4 is so much less soluble than the PbCl2, it is likely that by the time the Pb+2 has been removed to form PbSO4, the concentration of the Pb+2 will be so small that it is unlikely to be further affected by the Cl- ion.