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Name: Lisa
Status: educator
Grade: 9-12
Location: CA
Country: USA
Date: Winter 2011-2012

Question:
When doing a percent yield activity in lab, we use MgCl hexahydrate and CaSO4. How do we factor the mass of the water that is released during the reaction?

Replies:
Lisa,

Based on your question, I am not quite sure what the experiment is. Are you heating the hydrates and looking at the percent-yield of water removed during the heating? If so, then you would calculate the theoretical yield (using stoichiometry and the balanced chemical equation: MgCl2.6H2O --> MgCl2 + 6H2O) of water released, and compare it to the actual yield of water released in the experiment to get percent yield.

Greg (Roberto Gregorius) Canisius College


Be careful, the formula for the magnesium compound is MgCl2 (two chlorides not just one). That aside, you have to consider the amount of water released by the solution process. So dissolving MgCl2 *6H2O. reduces the concentration of the salt by the mass of water released by the solution process. Frequently the actual hydrated salt is only approximately equal to the nominal mass of the substance. You need to analyze the salt for the actual amount of Mg(+2) and Cl(-1) present, since most hydrates lose (or gain) some water just being exposed to the atmosphere.

Vince Calder


Lisa -

I looked up "percent yield" and it seems to be defined as yield = [amount of product actually produced] / [amount of product ideally expected].

The water released is probably not considered part of the intended product. Treat it as you would the other by-products of the reaction (analyze maybe, ignore for sure).

I could try to get more specific, but what you said is not quite enough for me to figure out what reaction you are doing or what product is expected to be created.

I might approach it like this: 1) write the whole reaction equation 2) balance the whole reaction in mole-coefficients, 3) choose a batch-size (perhaps from the moles of the mass to be weighed-out of one of your components) 4) find moles of all components in the balanced reaction 5) find masses of all components in the balanced reaction 6) yield = [mass of product an experimenter captures ] / [mass of product calculated in last step] 7) percent yield = yield * 100%

Steps (1) - (5) carry the water as part of the thorough accounting. You then have in hand everything you might need to know in the final stages, and if the calculations were done at a reasonably low price, that feels good and enables clear discussion. Steps (6) and (7) ignore the moles and mass of released water.

I would end up doing these calculations over and over, and also it really helps to line up the coefficients, moles, and grams of each component in a row or column,

so I would use a spreadsheet for sure. That is what gets the answers at the right price.

It seems to me there should be software, even freeware, for this kind of thing. But on a first look I am not finding exactly what I had in mind.

Jim Swenson


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