Calculating how much of the excess remains [ENDORSED]

[Sophie Krylova 2J]

I understand the concept of what it is and vaguely how I am supposed to find it, but can somebody show the actual calculations?

[Chem_Mod]

Do you have an example problem from the text that you are referring too? The general strategy for calculating excess is to find the limiting reagent and see how much of the excess product is consumed if all of the limiting reagent is reacted. Then you subtract the this mass from the starting mass of the excess reagent.

[Nicole Nikolov 1K]

Wouldn't you see how much of the excess product is consumed by comparing the molar ratios of the two (or more) reactants? You can never compare the masses of two different reactants; you would have to use moles, right?

[Kaylin Krahn 1I]

You would first find the limiting reactant and then use the ratio from the balanced chemical equation to find how much of the excess will be used. Then subtract how much will be used by the total excess...

So if you have two moles of each reactant and the ratio is 1:2, the amount of excess used is 1 and the amount of excess left would also be 1.

Trying to use simple numbers but hope that helps!

[Michelle Lee 2E]

If you want to find how much, in grams, of excess reactant remains, you can use stoichiometry. If you're given how much, in grams, there is of the reactants originally in the question, find the limiting reactant first by converting the elements/molecules to moles and looking at the molar ratios; look at how much of each is needed to create the product. If there are, for example, two reactants and one product, find the moles of both reactants and individually find out how much product is able to be produced from each reactant; the one that creates the least product is the limiting reactant.
Now in terms of finding how much of the excess reactant is left, you can work backwards from where you left off. Once you've found how much product can be produced by the limiting reactant, work backwards using stoichiometry to figure out how much of the excess reactant was actually used. When you figure that out, subtract it from the mass of the reactant originally given in the question. That should tell you how much excess there is.