# Example Problem of Mass Relations in Balanced Equations

A mass relation refers to the ratio of the mass of reactants and products to each other. In a balanced chemical equation, you can use the mole ratio to solve for mass in grams. You can use an equation to learn how to find the mass of a compound, provided you know the quantity of any participant in the reaction.

## Mass Balance Problem

The balanced equation for the synthesis of ammonia is 3 H2(g) + N2(g) → 2 NH3(g).

Calculate:

1. The mass in grams of NH3 formed from the reaction of 64.0 g of N2
2. The mass in grams of N2 required for form 1.00 kg of NH3

Solution:

From the balanced equation, it is known that:

1 mol N2 ∝ 2 mol NH3

Use the periodic table to look at the atomic weights of the elements and calculate the weights of the reactants and products:

1 mol of N2 = 2(14.0 g) = 28.0 g

1 mol of NH3 is 14.0 g + 3(1.0 g) = 17.0 g

These relations can be combined to give the conversion factors needed to calculate the mass in grams of NH3 formed from 64.0 g of N2:

Mass NH3 = 64.0 g N2 x 1 mol N2/28.0 g NH2 x 2 mol NH3/1mol NH3 x 17.0 g NH3/1 mol NH3

Mass NH3 = 77.7 g NH3

To obtain the answer to the second part of the problem, the same conversions are used in a series of three steps:

1. (1) grams NH3 → moles NH3 (1 mol NH3 = 17.0 g NH3)
2. (2) moles NH3 → moles N2 (1 mol N2 ∝ 2 mol NH3)
3. (3) moles N2 → grams N2 (1 mol N2 = 28.0 g N2)

Mass N2 = 1.00 x 103 g NH3 x 1 mol NH3/17.0 g NH3 x 1 mol N2/2 mol NH3 x 28.0 g N2/1 mol N2

Mass N2 = 824 g N2