Uncovering the Flaw in a Chemistry Lab Experiment

[Virogen]

Any ideas what the inherent flaw in this experiment is?

Homework Statement

In the lab (actual lab), we combined copper(II) sulfate pentahydrate with barium nitrate to form barium sulfate (precipitate). Every group got a 115% or higher yield than stoichiometry would predict.

Ba(NO3)2 + CuSO4 5H2O --> Cu(NO3)2 + BaSO4 + 5H2O

Mass of barium nitrate: 1 g
Mass of copper(II sulfate pentahydrate) 0.75 g

Moles of copper(II) sulfate pentahydrate: 0.75g/249.71g/mol = 0.00300 mol
Moles of barium nitrate: 1g/261.35 g/mol = 0.00383 mol

The limiting reagent is thus copper(II) sulfate pentahydrate, and because of the 1:1 mole ratio, 0.00300 mol of barium sulfate should be produced.

0.00300 mol x 233.39 g/mol = 0.700g

However, all my students recorded at least 0.8 g... I'm very confused. It must have something to do with the water/pentahydrate?? But I can't see that mathematically if I try to remove the water completely. Any ideas what the flaw in this experiment is?

 

[Borek]

Your calculations are correct (I got 0.7011g, using exact values of molar masses):

Two things I can think of.

First, copper sulfate pentahydrate doesn't have an exact composition - exact number of water molecules depends on the temperature, humidity and the sample history. If memory serves me well it is quite possible that the sulfate you were using was not pentahydrate, but something like four-and-half-hydrate.

Second, how were you drying the barium sulfate? In analytical chemistry we roast it to the constant mass in the 600-800°C, lower temperatures don't guarantee it will be completely dry. If it contains water its mass will be higher than expected.

 

[Virogen]

Well it's only high school so we let them air dry overnight (and it's a low humidity climate). It seemed very dry and flaked easily. Combined with the idea that it may not all be pentahydrate could possibly account for it.

Do you have a better suggestion for a percent yield lab?

Also, what program did you use to check these calculations? It looks massively helpful!

 

[Borek]

Some lab sites I know about:

http://www.rsc.org/learn-chemistry/wiki/Lab:Practical_Chemistry
http://science-house.org/index.php/ctc
http://www.mrteverett.com/chemistry/labs/labs.asp

Unfortunately, none seems to have just a "yield" lab. I would also look for ideas in the Journal of Chemical Education. Also, just googling for "percent yield lab" gives plenty of hits.

Also, what program did you use to check these calculations? It looks massively helpful!

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. It is massively helpful by design

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