One of the most common chemistry problems is figuring out how much product you will form if you do a reaction. The starting point with all of these types of problems is the balanced chemical equation.

The balanced chemical equation tells you how many molecules of a reactant are needed to produce some number of molecules of product. For example, in the combustion of methanol

2CH₃OH(l) + 3O₂(g) -> 4H₂O(g) + 2CO₂(g)

To figure out amount of product from amount of reactant, start with the number of moles of the reactant. (You may be given weight rather than moles: use the molecular weight to convert between the two.) Next, write out the conversion factor to convert from reactant to product, and simply multiply.

For example, if we had 0.50 moles of methanol, how much water do we form? We form four water molecules (moles) for every two molecules (moles) of methanol, so

0.50 moles methanol * 4 moles water/2 moles methanol = 1.00 moles water formed.

In the same vein, we can ask how much oxygen we used to burn 0.50 moles of methanol. We use three oxygen molecules (moles) for every two molecules (moles) of methanol, so

0.50 moles methanol * 3 moles oxygen/2 moles methanol = 0.75 moles oxygen used.

Example: How many moles of nitrogen do we form if we react 0.35 moles of N₂H₄ in the reaction below? (This is the reaction used in some missile fuels.)

2N₂H₄(l) + N₂O₄(l) -> 3N₂(g) + 4H₂O(g)

Solution: We have 0.35 moles of N₂H₄. For every two molecules (moles) of N₂H₄ we form three molecules (moles) of nitrogen, so

0.35 moles N₂H₄ * 3 moles N₂/2 moles N₂H₄ = 0.53 moles N₂ formed.

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