Chemical formulae, equations, and calculations

What is relative atomic mass (Ar)?

The average mass of all the isotopes of that element

Where is the relative atomic mass located on each element?

The top number

How do you find the relative formula mass which is the same as relative molecular mass (Mr)?

number of atoms of that element x their relative atomic mass

What is the unit for the amount of substance?

mol

What is the equation linking mass, Mr and moles

moles = mass / Mr

What is the molar ratio and what is the rule?

The ratio between the moles of the reactant and product in a balanced equation.

• Just the big number on the left side of the element, never the subscript

e.g. N₂ + 3H₂ —> 2NH₃

molar ratio : 1:3:2

What is percentage yield?

The actual yield of a reaction compared to the maximum theoretical yield

(meaning how much product you produce compared with how much you thought you should produce based on your calculations)

What is the formula for calculating percentage yield?

% yield = (actual yield/ theoretical yield) x 100

Suggest 3 reasons why the percentage yield may be less than 100%

1. Reactants may not all react (e.g. slow reaction or reaches equilibrium)

2. side reactions, maybe other products produced instead

3. products may be lost (e.g. gases may float off or some solids left on the filter paper)

How can you obtain formula of metal oxides experimentally? Describe example of aluminium oxide.

Through electrolysis

e.g. Aluminium oxide (2Al₂O₃ —> 4Al + 3O₂)

Anode - O₂ would be formed (2O^2- —> O₂ + 4e^-) (Oxidised - loss of electrons)

Cathode - Al would form (Al³⁺ + 3e^- —> Al) (Reduced - gain of electrons)

How can you obtain formula of aqueous solutions? And what are the rules?

Electrolysis

• H⁺ and OH^- ions are always present

• OH^- ions are always oxidised at the anode unless a halide ion (group 7) is present in solution

• H⁺ ions are always reduced at the cathode unless a less reactive element than H is present (e.g. Cu²⁺)

What is water of crytallisation?

It is the water molecules which are incoporated in a crystal lattice

What is a hydrated salt?

When the solid crystal contains water of crystallisation

What is an anhydrous salt?

When the solid crystal does NOT contain water of crystallisation

What is the empirical formula and how do you find it?

It is the smallest whole number ratio of atoms of each element within a compound

e.g. empirical formula of N₂O₄ would be NO₂

• Find out number of moles with moles = mass/ Mr

• Then divide each mole number by the smallest one out them to find the molar ratio.

• Substitute ratio into compound

What is the molecular formula and how do you find it?

It is the exact number of atoms of each element within a compound

• Use the empirical formula

• molecular mass = n x empirical formula

• n = the subscript found in the empirical formula

e.g. CH₂O x 2 —> C₂H₄O₂

State the equation linking concentration, moles and volume

concentration = moles / volume

What is the unit for volume?

dm³

What is the unit for concentration?

mol / dm³

What is 1 dm³ in m³

1 dm³ = 1000 cm³

State the equation linking number of moles and the volume of a gas

volume of gas = number of moles x 24

What are the conditions needed to use the equation volume of gas = moles x 24

• Room temperature

• Room pressure