Calculations Involving Moles

Mole Calculations

What is a mole?

The amount of substance that contains the same number of particles as there are carbon atoms in exactly 12 g of carbon-12 (¹²C).

What is the unit for amount of substance?

The mole (mol).

What symbol is used for amount of substance?

n.

How many particles are present in one mole?

6.02 × 10²³ particles.

What types of particles can be counted using moles?

Atoms, molecules, ions, electrons or other specified particles.

Why are moles used in chemistry?

Atoms, molecules and ions are extremely small and occur in very large numbers, so moles provide a convenient way of measuring amounts of particles.

Why must you identify the species when carrying out mole calculations?

The same mass can contain different amounts in moles depending on whether you are referring to atoms, molecules or ions.

Does 1 mol of oxygen atoms have the same mass as 1 mol of oxygen molecules?

No. 1 mol of O atoms has a mass of 16.0 g, whereas 1 mol of O₂ molecules has a mass of 32.0 g.

What is the amount in moles of 1.0 g of oxygen atoms, O?

1.0 ÷ 16.0 = 0.0625 mol.

What is the amount in moles of 1.0 g of oxygen molecules, O₂?

1.0 ÷ 32.0 = 0.0313 mol.

What is the amount in moles of 1.0 g of carbon dioxide molecules, CO₂?

1.0 ÷ 44.0 = 0.0227 mol.

What equation is used to calculate amount in moles from mass?

n = m/M.

What does n represent in n = m/M?

Amount of substance in mol.

What does m represent in n = m/M?

Mass in grams (g).

What does M represent in n = m/M?

Molar mass in g mol⁻¹.

How do you calculate moles from mass?

Divide the mass by the molar mass: n = m/M.

How do you calculate mass when the amount in moles and molar mass are known?

m = n × M.

How do you calculate molar mass when mass and amount in moles are known?

M = m/n.

What is molar mass?

The mass of one mole of a substance.

What are the units of molar mass?

g mol⁻¹.

What information is needed to calculate the molar mass of a compound?

The relative atomic masses and the chemical formula of the compound.

What is the amount of substance in 6.51 g of sodium chloride, NaCl, if M = 58.5 g mol⁻¹?

n = 6.51 ÷ 58.5 = 0.111 mol.

What equation should you use to find the mass of a substance from its number of moles?

m = n × M.

What is the mass of 0.263 mol of hydrogen iodide, HI, if M = 127.9 g mol⁻¹?

m = 0.263 × 127.9 = 33.6 g.

What equation should you use to find molar mass from mass and moles?

M = m/n.

A sample of 0.284 mol has a mass of 17.8 g. What is its molar mass?

M = 17.8 ÷ 0.284 = 62.7 g mol⁻¹.

What is the key triangle relationship between mass, moles and molar mass?

m = nM, n = m/M and M = m/n.

What should you check before starting a mole calculation?

Identify exactly which species or particles the question refers to.

Why is it important to distinguish between atoms and molecules in mole calculations?

A molecule can contain several atoms, giving it a different molar mass from an individual atom.

What should you clearly state if there could be ambiguity in a mole calculation?

The formula and name of the species being considered.

How many moles are in 8.00 g of sulfur atoms, S, if Aᵣ(S) = 32.1?

n = 8.00 ÷ 32.1 = 0.249 mol.

What is the molar mass of sulfur dioxide, SO₂, if S = 32.1 and O = 16.0?

32.1 + (2 × 16.0) = 64.1 g mol⁻¹.

How many moles are in 8.00 g of sulfur dioxide, SO₂?

n = 8.00 ÷ 64.1 = 0.125 mol.

What is the molar mass of the sulfate ion, SO₄²⁻?

32.1 + (4 × 16.0) = 96.1 g mol⁻¹.

How many moles of sulfate ions, SO₄²⁻, are in 8.00 g?

n = 8.00 ÷ 96.1 = 0.0832 mol.

What is the relationship between moles and number of particles?

Number of particles = amount in moles × 6.02 × 10²³.

How do you calculate the number of particles from an amount in moles?

Multiply the amount in moles by 6.02 × 10²³.

How do you calculate the amount in moles from the number of particles?

Divide the number of particles by 6.02 × 10²³.

What is a common mistake when using moles?

Not stating whether the particles are atoms, molecules, ions or electrons.