Calculations Using Reacting Masses

Mole Calculations

What does a balanced chemical equation tell you about reacting amounts?

The coefficients give the mole ratio in which reactants react and products form.

Why can a balanced equation be used to calculate reacting masses?

It gives the mole ratio between reactants and products, which can be converted into masses using molar masses.

What must remain constant when the amounts or masses in a reaction are changed?

The ratio between the reacting substances shown by the balanced equation.

What is the balanced equation for the manufacture of ammonia?

N₂ + 3H₂ → 2NH₃.

What is the mole ratio N₂ : H₂ : NH₃ in N₂ + 3H₂ → 2NH₃?

1 : 3 : 2.

How many moles of hydrogen react with 1 mol of nitrogen?

3 mol of H₂.

How many moles of ammonia form from 1 mol of nitrogen?

2 mol of NH₃.

What masses react in N₂ + 3H₂ → 2NH₃?

28.0 g N₂ + 6.0 g H₂ → 34.0 g NH₃.

What are the main steps for calculating a reacting mass?

1. Write the balanced equation. 2. Calculate molar masses. 3. Convert the known mass to moles using n = m/M. 4. Use the mole ratio from the equation. 5. Convert the required moles back to mass using m = nM.

What equation converts mass into moles?

n = m/M.

What equation converts moles into mass?

m = nM.

Where do you obtain the mole ratio for a reacting-mass calculation?

From the coefficients in the balanced chemical equation.

Why should you not directly compare masses using the coefficients in an equation?

The coefficients represent ratios of particles or moles, not ratios of masses.

For SO₃ + H₂O → H₂SO₄, what is the mole ratio SO₃ : H₂SO₄?

1 : 1.

What is the molar mass of SO₃?

80.1 g mol⁻¹.

What is the molar mass of H₂SO₄?

98.1 g mol⁻¹.

How many moles are present in 75.0 g of H₂SO₄?

n = 75.0 ÷ 98.1 = 0.765 mol.

How many moles of SO₃ are required to form 0.765 mol of H₂SO₄?

0.765 mol because the mole ratio is 1 : 1.

What mass of SO₃ is required to form 75.0 g of H₂SO₄?

m = 0.765 × 80.1 = 61.2 g.

For 2NH₃ + H₂SO₄ → (NH₄)₂SO₄, what is the mole ratio NH₃ : (NH₄)₂SO₄?

2 : 1.

What is the molar mass of NH₃?

17.0 g mol⁻¹.

What is the molar mass of (NH₄)₂SO₄?

132.1 g mol⁻¹.

How many moles are present in 100 g of (NH₄)₂SO₄?

100 ÷ 132.1 = 0.757 mol.

How many moles of NH₃ are needed to form 0.757 mol of (NH₄)₂SO₄?

2 × 0.757 = 1.51 mol.

What mass of NH₃ is needed to form 100 g of (NH₄)₂SO₄?

1.51 × 17.0 = 25.7 g.

How can reacting-mass data be used to determine a formula or equation?

Convert the measured masses to moles and find the simplest whole-number mole ratio.

What are the steps for determining a formula from reacting masses?

Calculate the moles of each relevant substance, divide to obtain the simplest whole-number ratio, then use the ratio to determine the formula.

What is a hydrate?

A compound containing water of crystallisation.

What is water of crystallisation?

Water molecules incorporated into the crystal structure of a compound.

How is a hydrated compound represented in a formula?

The compound formula is followed by a dot and the number of water molecules, e.g. CuSO₄·5H₂O.

What does CuSO₄·5H₂O mean?

Each formula unit of copper(II) sulfate is associated with five water molecules.

What is the molar mass of Na₂CO₃·10H₂O?

286.1 g mol⁻¹.

How many moles are in 16.7 g of Na₂CO₃·10H₂O?

16.7 ÷ 286.1 = 0.0584 mol.

How many moles are in 3.15 g of water?

3.15 ÷ 18.0 = 0.175 mol.

What is the simplest mole ratio of 0.0584 mol Na₂CO₃·10H₂O to 0.175 mol H₂O?

1 : 3.

What equation describes the partial dehydration of Na₂CO₃·10H₂O in the example?

Na₂CO₃·10H₂O → Na₂CO₃·7H₂O + 3H₂O.

How can experimental reacting masses distinguish between two possible reaction equations?

Convert the measured masses into moles and compare their mole ratio with the ratios predicted by the possible equations.

An oxide of copper produces 17.6 g Cu and 2.56 g H₂O when reduced with hydrogen. How many moles of Cu form?

17.6 ÷ 63.5 = 0.277 mol.

How many moles of H₂O form from 2.56 g?

2.56 ÷ 18.0 = 0.142 mol.

What is the approximate mole ratio Cu : H₂O in this experiment?

0.277 : 0.142 ≈ 2 : 1.

Which copper oxide is identified by a Cu : H₂O ratio of 2 : 1?

Cu₂O.

What is the equation for reducing copper(I) oxide with hydrogen?

Cu₂O + H₂ → 2Cu + H₂O.

Why does the experimental evidence support Cu₂O rather than CuO?

Cu₂O produces Cu and H₂O in a 2 : 1 mole ratio, matching the experimental results.

What is an important skill when using mole ratios in reacting-mass calculations?

Choosing the correct substances from the equation and using the ratio in the correct direction.