Question

If 143 grams of chromium react with an excess of oxygen, as shown in the balanced chemical equation below, how many grams of chromium oxide can be formed? Please show all your work for the calculations for full credit.

4Cr + 3O2 yields 2Cr2O3
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Answer 1

You've got a balanced equation, so your first step is done. If you didn't have one, that would be the first thing to do.

Your limiting reactant is the 143 grams of Cr. Convert that to moles by multiplying the mass by \[\frac{1\text { mol Cr}}{\text{molar mass of Cr}}\]you can find the molar mass of Cr in a periodic table — it is just the number given for atomic mass, except the unit will be grams rather than atomic mass units.

Next, look at your balanced equation and find the stoichiometric ratio of how many moles of the product appear on the right hand side of the arrow to the number of moles of reactant on the left hand side of the arrow. Here we have 2 Cr2O3 on the right side, and 4 Cr on the left, so we end up with a stoichiometric ratio of 2:4 (or 1:2 in simplest form). In other words, we will have half as many moles of product as reactant.

Multiply the number of moles of reactant by the stoichiometric ratio to get the number of moles of product.

To get the number of grams of product, multiply the number of moles of product by the molecular mass of the product.