CH 101/Cooley's stoichiometry hints

From WolfWikis

Stoichiometry Problems

Here are some alternative ways to work these problems out. If you understand ratios, this should be much easier to follow along and understand than some previous methods. A couple of hints:

1. When you are given a compound, you can use the ratios of the subscripts to find out several of your answers. Here is an example using the compound Ca2S3. You can see that the there are 2 Calcium atoms for every 3 Sulfur atoms just by looking at the subscripts. In other words, the ratio of Calcium to Sulfur is 2:3, with can also be said 2/3. Knowing this can help with many different problems.

Example 1

If you have 5 moles of Sulfur, how many moles of Calcium do you have? You know that you have 2 moles of Ca for every 3 moles of S, or 2/3 ratio. Now multiply this ratio by the given information.

(2/3) x 5 moles of S = 3.33 moles of Ca

This method works only when you are going from moles of one atom to moles of another atom.

Here is a little trick for when you are given grams of a particular substance.

Example 2

Given 8g of Ca in the formula Ca2S3, how many grams of S do you have?

This can be done simply by cross multiplying. You will need to know the ratio of the two atoms, which we already found to be 2/3. Next, you will need to find the molar mass of each atom.

Mm of Ca is 40.04 g/mol

Mm of S is 32.07 g/mol

Using the ratios and molar masses you can lay out this problem and cross multiply and solve for the unknown or “X” like you did in algebra back in high school.

(8 g of Ca)x(96.21 g S) / (80.16g of Ca)x(X)

= 769.68 / 80.16

= 9.6 g of S