AQA A-Level Chemistry [Amount of Substance]

chem

Define the term relative atomic mass

The weighted mean mass of an atom of an element

Define the term relative molecular mass

This is the average mass of a molecule compared to 1/12th of the mass of an atom of carbon-12.

Define the term relative formula mass

This is the average mass of a formula unit compared to 1/12th of the mass of an atom of carbon-12. RFM is used for ionic and giant covalent substances.

What is the relative atomic mass of an element?

This is the weighted average mass of all of its naturally occurring isotopes

List the steps needed to calculate relative atomic mass

1. Multiply each isotope's relative isotopic mass by its isotopic abundance.
2. Sum these quantities.
3. Divide by the sum of the isotopic abundances.

Boron has two naturally occurring isotopes:
• 10^B has relative isotopic mass 10.0 and abundance 20.0%
• 11^B has relative isotopic mass 11.0 and abundance 80.0%
Calculate the relative atomic mass of boron to 1 decimal place.

Relative atomic mass: 10.8

Steps:
10^B → 10.0 × 20.0 = 200.0
11^B → 11.0 × 80.0 = 880.0

Total → 200.0 + 880.0 = 1

In a mass spectrum

what does the y-axis show?

In a mass spectrum

what does the x-axis show?

The mass spectrum shows neon has three isotopes:
• 20^Ne has a relative isotopic mass of 20 and a relative abundance of 57.0.
• 21^Ne has a relative isotopic mass of 21 and a relative abundance of 0.1.
• 22^Ne has a relative isotopic mass of 22 and a relative abundance of 5.6.
Calculate the relative atomic mass of neon to one decimal place.

Relative atomic mass: 20.2

Steps:
20^Ne → 20 × 57.0 = 1

What is the relative molecular mass of a compound?

The sum of the relative atomic masses of all the atoms in the compound's formula unit.
*For molecular compounds comprised of covalently bonded molecules

List the steps needed to calculate the relative formula mass.

1. Write down the compound's formula unit or molecular formula.
2. Identify the number of atoms of each element present in the formula.
3. For each element

Calculate the relative molecular mass of water.

Relative molecular mass: 18.0

Steps:

Formula: H20

H → 1.0 × 2 = 2.0
O → 16.0 × 1 = 16.0

Total → 16.0 + 2.0 = 18.0

What is the value of Avogadro's constant

and what does it represent?

How do you calculate the number of moles in a substance?

Number of moles = number of particles/Avogadro's constant

Calculate the number of moles in 2.71 x 10^24 molecules of chlorine.
Avogadro's constant = 6.022 x 10^23 mol-1

4.50 mol

Number of moles = (2.71x10^24)/(6.022x10^23)
= 4.50 mol

What does the molar mass represent?

The mass in grams of 1 mole of a given substance.

What is the equation linking the number of moles

mass in grams

What is the concentration of a solution measured in?

g/dm^3 or mol/dm^3

What is the equation linking mass

volume and concentration?

What is the equation linking moles

volume and concentration?

How do you go from cm^3 to dm^3?

Divide the value in cm^3 by 1

How do you go from g/dm^3 to mol/dm^3?

Multiply the g/dm^3 value by the Mr of the solute

Titration experiments can be used to determine the concentration of a solution. List the steps to calculate the concentration.

1. Write a balanced equation for the reaction.
2. Use the titration volumes and a known concentration to calculate the moles of one reactant.
3. Use stoichiometry to relate moles of this reactant to the moles of the reactant whose concentration is unknown.
4. Divide the moles by the volume to calculate the unknown concentration.

What does the kinetic theory describe?

The constant motion of gas particles

What assumptions does the kinetic theory use?

1. Gas particles move rapidly and randomly.
2. The volume of the actual gas particles is negligible compared to empty space between them.
3. There are no intermolecular attractions or repulsions.
4. Collisions between particles are elastic - no energy is lost.
5. The average kinetic energy of particles depends only on their temperature.

A hypothetical gas obeying all these assumptions is called an ideal gas.

What is the ideal gas equation?

pV = nRT
Where:
p = pressure

What is the empirical formula?

The simplest whole number ratio of atoms of each element in a compound

What is the molecular formula?

The actual numbers of atoms in a molecule.