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Mass Relations in Balanced Equations Example Problem

Finding Mass of Reagents and Products

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The divalent nitrogen molecule is formed by a triple bond between two atoms of nitrogen.

The divalent nitrogen molecule is formed by a triple bond between two atoms of nitrogen.

Ben Mills
Problem

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

Calculate:
a. the mass in grams of NH3 formed from the reaction of 64.0 g of N2
b. 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 of the atomic weights of the elements to 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) grams NH3 → moles NH3 (1 mol NH3 = 17.0 g NH3)

(2) moles NH3 → moles N2 (1 mol N2 ∝ 2 mol NH3)

(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

Answer

a. mass NH3 = 77.7 g NH3
b. mass N2 = 824 g N2

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