Question

Converting between quantities

To convert from a given quantity of one reactant or product to the quantity of another reactant or product:

First, convert the given quantity to moles. Use molar masses to convert masses to moles, and use Avogadro's number (6.02×1023particles per mole) to convert number of particles to moles.

Next, convert moles of the given reactant or product to moles of the desired reactant or product using the coefficients of the balanced chemical equation. For example, in the chemical equation

2H2+O2→2H2O

the coefficients tell us that 2 mol of H2 reacts with 1 mol of O2 to produce 2 mol of H2O.

Finally, convert moles of the desired reactant or product back to the desired units. Again, use molar masses to convert from moles to masses, and use Avogadro's number to convert from moles to number of particles.

Reaction of hydrogen and nitrogen to form ammonia

Hydrogen gas, H2, reacts with nitrogen gas, N2, to form ammonia gas, NH3, according to the equation

3H2(g)+N2(g)→2NH3(g)

NOTE: Throughout this tutorial use molar masses expressed to five significant figures.

1, How many moles of NH3 can be produced from 21.0 mol of H2 and excess N2?

2, How many grams of NH3 can be produced from 3.86 mol of N2 and excess H2.

3, How many grams of H2 are needed to produce 14.96 g of NH3?

4, How many molecules (not moles) of NH3 are produced from 6.08×10⁻⁴ g of H2?

Answer 1

The reaction is,

3H2(g)+N2(g)→2NH3(g)

According to the stoichiometry of the reaction 3 moles of H2 reacts with 1 mole of N2 to produce 2 moles of NH3

1)

3 moles of H2 produces 2 moles of NH3

=> 1 mole of H2 produces (2/3) moles of NH3

=> 21 moles of H2 produces (2/3) x 21 = 14 moles of NH3

2)

1 mole of N2 produces 2 moles of NH3

=> 3.86 moles of N2 produces 3.86 x 2 = 7.72 moles of NH3

3)

Molar Mass of NH3 = 17 g / mol

=> Moles of N2 = 14.96 / 17 = 0.88 moles

2 moles of NH3 is produced by 3 moles of H2

=> 0.88 moles of NH3 is produced by (3/2) x 0.88 = 1.32 moles of H2

Molar Mass of H2 = 2 g / mol

=> Mass of H2 - 1.32 x 2 = 2.64 g

4)

Moles of H2 = 6.08 x 10^-4 / 2 = 3.04 x 10^-4 moles

3 moles of H2 produces 2 moles of NH3

=> 3.04 x 10^-4 moles of H2 produces (2/3) x 3.04 x 10^-4 = 2.03 x 10^-4 moles

Number of molecules = 2.03 x 10^-4 x 6..022 x 10^23 = 1.22 x 10^20 molecules