Mole to mole stoichiometry
Use the following equation: 2Fe + 3S —-> Fe2S3
to calculate how many moles of sulfur will react with 1.42 moles of Fe.
From the question, you have to ask yourself the following questions:
- which chemicals are being compared to each other? S and Fe.
- is the equation balanced? Yes
- what am I asked to calculate? Moles of S
- what’re the two ratios between the two chemicals?
- 2 mol Fe/3mol S
- 3 mol S/ 2 mol Fe
Since you are given moles of Fe, but asked to calculate moles of S. That means you have to pick the ratio that will allow you cancel out moles of Fe, but leave out moles of S. If you picked correctly, you will notice that the ratio is (3 mol S/2 mol Fe). As you can see, the 3 mol S is on top of the ratio, while the 2 mol Fe is at the bottom of the ratio. If you multiply the 1.42 mol Fe by the ratio (3 mol S/2 mol Fe), the moles of Fe will cancel each other out.
If we set it up like we did during unit conversions, we will get something like this:
1.42 mol Fe x 3 mol S/2 mol Fe = 2.13 mol S. There you have it. Based on the chemical equation, 2.13 moles of S will react with 1.42 moles of Fe.
As you know, you can’t go to a scale and weigh S in moles. So, what should you do? You have to convert moles of S to mass of S. To convert, you multiply the moles of S by the molar mass of S. From the periodic table, the molar mass of S is 32.07 g/mol. Therefore, the mass of 2.13 mol of S is: 2.13
mol S x 32.07 g/ mol S = 68.31 g S. Now, we can go and weigh 68.31 g of S on a scale.
To learn how to use a chemical equation to convert from mass of one chemical to another, click here
To learn how to determine the limiting reactant in a chemical reaction, click here.