These are worked chemistry problems showing how to calculate the number of moles of reactants or products in a balanced chemical equation.

### Mole Relations Problem #1

Determine the number of moles of N_{2}O_{4} needed to react completely with 3.62 mol of N_{2}H_{4} for the reaction 2 N_{2}H_{4}(l) + N_{2}O_{4}(l) → 3 N_{2}(g) + 4 H_{2}O(l).

### How to Solve the Problem

The first step is to check to see that the chemical equation is balanced. Make sure the number of atoms of each element are the same on both sides of the equation. Remember to multiply the coefficient by all atoms following it. The coefficient is the number in front of a chemical formula. Multiply each subscript only by the atom right before it. The subscripts are the lower numbers found immediately following an atom. Once you verify the equation is balanced, you can establish the relationship between the number of moles of reactants and products.

Find the relation between moles of N_{2}H_{4} and N_{2}O_{4} by using the coefficients of the balanced equation:

2 mol N_{2}H_{4} is proportional to 1 mol N_{2}O_{4}

Therefore, the conversion factor is 1 mol N_{2}O_{4}/2 mol N_{2}H_{4}:

moles N_{2}O_{4} = 3.62 mol N_{2}H_{4} x 1 mol N_{2}O_{4}/2 mol N_{2}H_{4}

moles N_{2}O_{4} = 1.81 mol N_{2}O_{4}

### Answer

1.81 mol N_{2}O_{4}

### Mole Relations Problem #2

Determine the number of moles of N_{2} produced for the reaction 2 N_{2}H_{4}(l) + N_{2}O_{4}(l) → 3 N_{2}(g) + 4 H_{2}O(l) when the reaction begins with 1.24 moles of N_{2}H_{4}.

### Solution

This chemical equation is balanced, so the molar ratio of reactants and products may be used. Find the relation between moles of N_{2}H_{4} and N_{2} by using the coefficients of the balanced equation:

2 mol N_{2}H_{4} is proportional to 3 mol N_{2}

In this case, we want to go from moles of N_{2}H_{4} to moles of N_{2}, so the conversion factor is 3 mol N_{2}/2 mol N_{2}H_{4}:

moles N_{2} = 1.24 mol N_{2}H_{4} x 3 mol N_{2}/2 mol N_{2}H_{4}

moles N_{2} = 1.86 mol N_{2}O_{4}

### Answer

1.86 mol N_{2}

### Tips for Success

The keys to getting the correct answer are:

- Make sure the chemical equation is balanced.
- Use the coefficients in front of compounds to get molar ratios.
- Check to make sure you use the appropriate number of significant figures for atomic masses and report mass using the correct number of figures.