# Calculating amount of precipitate that will form

20,0ml of 0,150M Zn(NO3)2 are mixed with 20,0ml of 1,00 x10^-4
M Na2S. Calculate the amount of milligrams of
ZnS (s) that will precipitate. (Ksp = 1,6 x10^-24
M^2 )

a) 2,68mg
b) 0,347mg
c) 4,85mg
d) 0,194mg

There are zero examples in my textbook or notes that deal with calculating the amount of precipitate that will form. Can someone help?

symbolipoint
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Does the problem description give a large excess of one of the reactants? What can this do the the equilibrium of the reaction? Is the product very very insoluble based on the size of the solubility product constant?

1) Yes,

2) Makes it small

3) Yes

I think I got it. I calculated 4.85mg by finding the new concentrations with the new volume and then using the smaller concentration to find the amount in grams.

chemisttree
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Gold Member
Show your calculations. Remember you only have a 25% chance if you quess the answer.

[Zn] = (0.15M x 0.02L)/(0.04L) = 0.075M

= (1x10^-4M x 0.02L)/(0.04L) = 5 x 10^-5M

Then multiplying 5 x 10^-5M by molar mass of ZnS and finding the amount of grams in 0.04L I got 4.85mg.

chemisttree
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Gold Member
Try again. Why are you multiplying a concentration by a formula wt?

Units would be grams/liter...

chemisttree
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If you have already decided that one of the reagents is limiting and that the concentration of ZnS in solution is insignificant (something times 10^-23 mol/L, I believe), then the amount of precipitate will essentially be equal to the number of moles of the limiting reagent.

If you have already decided that one of the reagents is limiting and that the concentration of ZnS in solution is insignificant (something times 10^-23 mol/L, I believe), then the amount of precipitate will essentially be equal to the number of moles of the limiting reagent.

Isn't that what I did? S is limiting, so I took the mass of that, which is 4.85mg.

chemisttree
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Gold Member
moles S = (1 X 10^-4 mol/L)*(0.02L) = 2 X 10^-6 mol S

mass of S (g) = (2 X 10^-6 mol S)*(FW ZnS)

mass of ZnS (mg) = 1000 * mass ZnS (g)

You found the amount in grams in 0.02 L. The final volume is 0.04 L.

Last edited:
Ah I see it now. Thanks. (Now how about that other thread? :P)