Limiting reagent and grams of product
Type the balanced equation and how much of each reactant you have, in grams or in moles: out comes which one runs out first, how much product forms, how much of the other is left over and, if you know it, the percentage yield.
The ratio that decides, and the mistake everybody makes
The limiting reagent is not the one that weighs least, and this is where it goes wrong almost every time. You turn the grams into moles, divide by the coefficient in the equation and the smallest quotient wins: that is the reactant that runs out first, and it rules everything else. A hundred grams of oxygen is many more moles than a hundred grams of iron, so «I have more of it in grams» does not mean «some will be left».
The other common mistake is dividing the grams by the coefficient instead of the moles: a number comes out, it looks like an answer, and it is wrong. The table above shows all three steps in a row, moles, quotient and what is left over, precisely because an exercise is corrected by looking at where the sum took the wrong turn, not by looking at the final figure.
The equation has to be balanced already
With an unbalanced equation the sum would come out all the same, and it would be wrong with no warning at all: the coefficients are exactly the numbers you divide by. That is why the page counts the atoms on both sides before working anything out, and if they do not match it says so, listing which element is off and by how much.
If the equation you have is not balanced, the step to take first is Balance a chemical equation, which puts the coefficients in and checks them by counting the atoms again. And if all you need is the weight of a formula, the short way is Molar mass.
How much is left over, and the yield
The second half of nearly every exercise is how much of the other reactant is left over, and it comes from the same quotient: the moles used are the smallest quotient times the coefficient, and what remains is the difference. The table already gives it in grams, which is how the book asks for it.
If you actually weighed the product, typing how many grams came out shows the percentage yield. Above 100% is not a good sign and the page says so: nearly always it means the weighed product still held water or solvent, or that an amount was typed wrong.
Three pages talking about the same chemistry
The atomic weights on this page are the same as the ones on the page that works out molar mass, copied across and not rewritten: two tables differing by a hundredth would give two pages of the same site two different answers about the same formula, and it is the kind of error nobody spots until somebody compares them.
For the same reason the atom counting is compared with the page that balances equations: the test takes its function from the published page and demands that the two programs count the same on thousands of formulas. Brackets and hydrates (for example CuSO4·5H2O) are typed the way the book writes them.