Stoichionetry (C3)

IGCSE Chemistry: C3.2: Relative masses of atoms and molecules C3.3: The mole and Avogadro constant

What is relative atomic mass (A_r)?

The average mass of the isotopes of an element compared to 1/12th of the mass of an atom of ¹²C, it’s shown on the atomic symbol, and it’s always bigger than the atomic number

What is relative molecular mass (Mᵣ)?

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

What are the calculations needed?

What is the concentration of a substance, and what can it be measured in?

The amount of solute dissolved in a measured volume of solution. It can be measured in g/dm³ or mol/dm³

What is a mole?

A unit that can be used to measure the amount of substance

What is Avogadro’s number?

6.02 x 10²³

How many particles does a mole have?

One mole contains 6.02 x 10²³ particles

What is the molar mass for an element and for a compound?

For an element: the same as relative atomic mass (Aᵣ) but in grams

For a compound: the same as relative molecular mass (Mᵣ) but in grams

What is the equation for moles?

How do you calculate the number of particles?

number of particles = moles (n) × Avogadro's number (6.02 × 10²³)

What is the volume of 1 mol of gas at room temperature?

24dm³

How do you convert cm³ to dm³

Divide by 1000

How do you convert dm³ to cm³?

Multiply by 1000

What are the calculations in a gas?

What is a limiting reactant?

The reactant in a chemical reaction that is used up first and determines the amount of product that is formed

By working out the mass of the limiting reactant, what else can we work out?

The moles

What is the process to calculate liming reactants?

1. Balance the chemical equation

2. Calculate the number of moles of each reactant

3. Divide the number of moles of each reactant by the coefficients from the balanced equation

4. The smaller result is the limiting reactant

(If there is no coefficient, the smaller amount of moles is the limiting reactant)