Stoichiometry Module: General Stoichiometry

Stoichiometric Factors

In a laboratory experiment, amounts of substances are usually measured on a scale (mass) or with some type of measuring glassware (volume). While adding a step to a stoichiometric problem, this doesn't make the problem more difficult as long as you remember the coefficients in a balanced chemical equation relate the number of molecules (or number of moles) that react, NOT the mass or volume. Mass can always be converted to moles using the molecular weight of the substance and volume can always be converted to moles using the concentration of the solution.

How many grams of oxygen are needed to react with 16 g of methane (the molecular weights of methane and oxygen are 16 g/mol and 32 g/mol respectively)?

This problem can be mapped out much like the last one. In mapping out a problem, it is useful to follow these steps (Clicking on the step number will update a map that is shown after the three steps - you can see it if you scroll down the page):

Step 1: Start in the middle of the map. The balanced chemical equation will always relate moles of one material to moles of another material. In the problem, you are given an amount (16 g) of methane and asked to relate it to an amount of oxygen. Thus, in the center of the map, there will be a step relating moles of methane to moles of oxygen. This relationship is made using the appropriate stoichiometric factor (or mole ratio).

Step 2: If the given information is in a unit other than moles, the first step in the problem will be to convert this amount to moles. Thus, the grams of methane must be converted to moles of methane. This conversion requires the use of the molecular weight of methane.

Step 3: If the problem asks for the solution in a unit other than moles, the amount asked for must be converted from moles to the desired units. Thus, the moles of oxygen must be converted to grams of oxygen. Again, this conversion requires the use of the molecular weight of oxygen.

Using the map as a guide, continue to solve the problem numerically (if you need help for any step, you can click on the arrow on the map for that step):

How many grams of oxygen are needed to react with 16 g of methane (the molecular weights of methane and oxygen are 16 g/mol and 32 g/mol respectively)?

	8 g	16 g	32 g	64 g