3. Quantitative chemistry

Law of conservation of mass

Mass cannot be created or destroyed in a chemical reaction.

4 in CH₄

There are four hydrogen atoms in the molecule.

3 in 3H₂O

There are three water molecules.

Mr

Relative formula mass.

Relative formula mass

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

Calculating relative formula mass

Add together the relative atomic masses of all atoms in the formula.

Percentage by mass of an element

(Ar of the element × number of atoms ÷ Mr of the compound) × 100.

Mass of a metal when it reacts with oxygen

It increases because it gains oxygen.

Mass of a metal carbonate when it thermally decomposes

It decreases as carbon dioxide is lost.

Uncertainty

An estimate of the range within which the true value lies.

Percentage uncertainty

(uncertainty ÷ measured value) × 100.

Chemical symbol for moles

n.

Number of particles in a mole

Avogadro's constant.

Particles in one mole of a substance

6.022 × 10²³.

Limiting reactant

The reactant that is completely used up in a reaction.

Limiting reactant in a chemical reaction

It is used up first, stopping the reaction.

Excess reactant

The reactant that is not limiting in a chemical reaction.

Solution

A mixture of a solute dissolved in a solvent.

Concentration of a solution

Mass of solute ÷ volume of solution.

Standard unit for measuring concentration

g/dm³.

Relationship between cm³ and dm³

1 dm³ = 1000 cm³.

Mass of a solute and volume of a solvent relation to concentration

Higher mass or lower volume = higher concentration.

Equipment to measure known volume in titration

Pipette.

Equipment to measure volume of reactant in titration

Burette.

Volumes measured in titration

To two decimal places.

Endpoint of a titration

When the indicator changes colour showing neutralisation.

Indicator

A substance that changes colour at different pH values.

White tile in titration

To clearly see the colour change.

Concordant data

Titres that are within 0.10 cm³ of each other.

Decimal places for titres

Two decimal places.

Yield of a reaction

The amount of product obtained compared to the maximum possible.

Reasons for not achieving expected yield

Reversible reactions, side reactions, loss during transfer.

Percentage yield formula

(actual yield ÷ theoretical yield) × 100.

Atom economy

How much of the reactants end up as useful products.

Importance of atom economy in industry

Reduces waste and costs.

Atom Economy formula

(RFM of desired products ÷ RFM of all products) × 100.

Room temperature and pressure

20°C and 1 atmosphere.

Volume of one mole of gas at room temperature and pressure

24 dm³.