Mole Relations in Balanced Equations

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 N₂O₄ needed to react completely with 3.62 mol of N₂H₄ for the reaction 2 N₂H₄(l) + N₂O₄(l) → 3 N₂(g) + 4 H₂O(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 N₂H₄ and N₂O₄ by using the coefficients of the balanced equation:

2 mol N₂H₄ is proportional to 1 mol N₂O₄

Therefore, the conversion factor is 1 mol N₂O₄/2 mol N₂H₄:

moles N₂O₄ = 3.62 mol N₂H₄ x 1 mol N₂O₄/2 mol N₂H₄

moles N₂O₄ = 1.81 mol N₂O₄

Answer

1.81 mol N₂O₄

Mole Relations Problem #2

Determine the number of moles of N₂ produced for the reaction 2 N₂H₄(l) + N₂O₄(l) → 3 N₂(g) + 4 H₂O(l) when the reaction begins with 1.24 moles of N₂H₄.

Solution

This chemical equation is balanced, so the molar ratio of reactants and products may be used. Find the relation between moles of N₂H₄ and N₂ by using the coefficients of the balanced equation:

2 mol N₂H₄ is proportional to 3 mol N₂

In this case, we want to go from moles of N₂H₄ to moles of N₂, so the conversion factor is 3 mol N₂/2 mol N₂H₄:

moles N₂ = 1.24 mol N₂H₄ x 3 mol N₂/2 mol N₂H₄

moles N₂ = 1.86 mol N₂O₄

Answer

1.86 mol N₂

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.