When conducting experiments, chemists do not have measuring tools that report units of moles. It is more reasonable to talk about amounts in units of mass, which can be measured using scales and balances. The mass in grams of one mole of a substance. The relationship between moles and mass, called molar mass, is specific to each substance. Let’s see how you can apply the molar ratios you have been using with molar mass to solve problems with units of grams.

Let’s summarize how you solved the problem. You read the problem to identify which two chemicals were being related to each other. You looked to the balanced chemical equation to find the mole ratio between the two chemicals. Once you had the units of your starting chemical in moles, you used your coefficient ratio to convert to the chemical you wanted. Finally, you continued your dimensional analysis until you had your desired unit.

Since you are now familiar with the combustion of ethane, work on a couple more practice problems. Don’t let decimals distract you from your problem solving!

1. How many grams of ethane are required to react with 42.6 grams of oxygen gas?

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   Remember that the heart of stoichiometry is finding the right molar ratio to use based on the coefficients of the balanced equation. To use it you first have to get to moles of the chemical you are given.

2. If the combustion of ethane produced 205 grams of carbon dioxide, how many grams of water were also produced?

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   Remember that the only time you will use coefficients is when you are ready to convert between moles of one chemical to moles of another. This is the heart of stoichiometry.