Comparing Masses of Substances

Mole Calculation

What is relative atomic mass (Aᵣ)?

The weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12.

What symbol is used for relative atomic mass?

Aᵣ.

What isotope is used as the standard for relative atomic mass?

Carbon-12 (¹²C).

What fraction of the mass of a carbon-12 atom is used as the reference for relative atomic mass?

1/12.

Why is relative atomic mass usually not a whole number?

It is a weighted mean that takes into account the masses and abundances of the element's isotopes.

Does relative atomic mass have units?

No.

What is relative molecular mass (Mᵣ)?

The sum of the relative atomic masses of all the atoms in a molecule.

What symbol is used for relative molecular mass?

Mᵣ.

How do you calculate relative molecular mass?

Add together the Aᵣ values of all atoms in the molecular formula.

Does relative molecular mass have units?

No.

What types of substances is relative molecular mass used for?

Substances that exist as molecules.

What is the Mᵣ of CO₂ if C = 12.0 and O = 16.0?

12.0 + (2 × 16.0) = 44.0.

What is the Mᵣ of H₂SO₄ if H = 1.0, S = 32.1 and O = 16.0?

(2 × 1.0) + 32.1 + (4 × 16.0) = 98.1.

What is relative formula mass (Mᵣ)?

The sum of the relative atomic masses of all atoms shown in a formula unit.

When should the term relative formula mass be used instead of relative molecular mass?

For substances that do not exist as individual molecules, such as ionic compounds and giant structures.

How is relative formula mass calculated?

Add together the Aᵣ values of all atoms in the formula.

Does relative formula mass have units?

No.

What is molar mass?

The mass per mole of a substance.

What symbol is used for molar mass?

M.

What are the units of molar mass?

g mol⁻¹.

How is molar mass related to relative molecular or formula mass?

They have the same numerical value, but molar mass has units of g mol⁻¹.

If a substance has an Mᵣ of 44.0, what is its molar mass?

44.0 g mol⁻¹.

What is a mole?

The amount of substance containing 6.02 × 10²³ particles.

What is the equation relating amount in moles, mass and molar mass?

n = m/M.

What does n represent in n = m/M?

Amount of substance in moles.

What does m represent in n = m/M?

Mass of the substance in grams.

What does M represent in n = m/M?

Molar mass in g mol⁻¹.

How do you calculate amount in moles from mass?

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

How do you calculate mass from moles and molar mass?

m = nM.

How do you calculate molar mass from mass and moles?

M = m/n.

What is the molar mass of O₂?

32.0 g mol⁻¹.

How many moles are in 5.26 g of O₂?

n = 5.26/32.0 = 0.164 mol.

What is the molar mass of CH₄?

16.0 g mol⁻¹.

How many moles are in 4.0 g of CH₄?

n = 4.0/16.0 = 0.25 mol.

What is the molar mass of H₂O?

18.0 g mol⁻¹.

How many moles are in 100 g of H₂O?

n = 100/18.0 = 5.56 mol.

What is the molar mass of NH₄NO₃?

80.0 g mol⁻¹.

What is the Avogadro constant?

The number of particles in one mole of a substance.

What symbol is used for the Avogadro constant?

L.

What is the value of the Avogadro constant?

6.02 × 10²³ mol⁻¹.

What types of particles can be counted using the Avogadro constant?

Atoms, molecules, ions or other specified particles.

How many particles are present in one mole of any substance?

6.02 × 10²³ particles.

How many helium atoms are in 4.0 g of helium?

6.02 × 10²³ atoms.

How many CO₂ molecules are in 44.0 g of CO₂?

6.02 × 10²³ molecules.

How many nitrate ions are in 62.0 g of NO₃⁻?

6.02 × 10²³ nitrate ions.

How do you calculate the number of particles from moles?

Number of particles = amount in mol × Avogadro constant.

What is the equation for the number of particles?

N = nL.

How do you calculate moles from the number of particles?

n = N/L.

How do you calculate the number of particles from a given mass?

First calculate n = m/M, then calculate N = nL.

How do you calculate the mass from a given number of particles?

Calculate n = N/L, then calculate m = nM.

How many moles are in 1.25 g of water?

n = 1.25/18.0 = 0.0694 mol.

How many water molecules are in 1.25 g of water?

0.0694 × 6.02 × 10²³ = 4.18 × 10²² molecules.

What are the two steps for finding the number of molecules in a given mass?

Calculate the number of moles using n = m/M, then multiply the moles by 6.02 × 10²³.

What is the first step when finding the mass of a given number of particles?

Divide the number of particles by the Avogadro constant to find the number of moles.

What is the second step when finding the mass of a given number of particles?

Multiply the number of moles by the molar mass.

What equation can combine mass, molar mass and number of particles?

N = (m/M) × 6.02 × 10²³.

What is the difference between Aᵣ and Mᵣ?

Aᵣ refers to an individual element's atoms, whereas Mᵣ is calculated for an entire molecule or formula unit.

What is the difference between Mᵣ and molar mass?

Mᵣ has no units, while molar mass has units of g mol⁻¹.

What is the key conversion between microscopic particles and macroscopic amounts in chemistry?

1 mol = 6.02 × 10²³ particles.