Quantitative chemistry

Conservation of Mass

In a chemical reaction, no atoms are lost or made, so the mass of the products equals the mass of the reactants.

Relative Formula Mass (Mr)

The sum of the relative atomic masses (Ar) of all atoms in a compound’s formula

Uncertainty

The possible range of values within which the true value of a measurement lies.

Closed System

A system where no substances can enter or leave, so mass remains constant.

Open System

A system where substances like gases can escape, which may cause apparent mass changes.

Why does mass appear to change in some reactions?

If the reaction is in an open system, gases may enter (e.g. oxygen) or leave (e.g. carbon dioxide), changing the measured mass.

What is the relative formula mass (Mr) of CO₂?

➤ Carbon = 12, Oxygen = 16
➤ Mr = 12 + (2 × 16) = 44

What happens to mass in a closed system during a chemical reaction?

Mass is conserved – it stays the same.

How is uncertainty in a measurement estimated?

By giving a ± value based on instrument precision or repeat range.

(2 marks): A student reacts calcium carbonate with hydrochloric acid in an open flask. The mass appears to decrease. Why?

Carbon dioxide gas escapes from the flask into the air, so the measured mass decreases.

Mole

The amount of a substance containing 6.022 × 10²³ particles (Avogadro's number).

Avogadro's Constant

6.022 × 10²³ – the number of atoms, ions, or molecules in one mole of a substance.

Limiting Reactant

The reactant that is completely used up in a reaction, limiting the amount of product formed.

How do you calculate moles from mass and Mr?

Moles = Mass (g) ÷ Mr

Calculate the number of moles in 12 g of carbon (Mr = 12).

Moles = 12 ÷ 12 = 1 mole

How do you calculate mass from moles?

Mass = Moles × Mr

How can you use moles to balance an equation?

Convert masses to moles, divide by smallest value, and round to whole numbers.

Theoretical Yield

The maximum mass of product possible from a given amount of reactants.

Actual Yield

The mass of product actually obtained in a reaction.

Percentage Yield

A measure of how much product was actually obtained vs. what was expected.

% Yield = (Actual ÷ Theoretical) × 100

Atom Economy

A measure of how many of the atoms in reactants are converted to useful products.

Atom Economy = (Mr of desired product ÷ Total Mr of all products) × 100

What is the percentage yield if the theoretical yield is 20 g and actual yield is 15 g?

(15 ÷ 20) × 100 = 75%

Why is atom economy important?

It reduces waste, improves sustainability, and makes processes more cost-effective.

Concentration (g/dm³)

➤ The amount of solute dissolved in a volume of solution.
➤ Concentration = Mass ÷ Volume

Molar Concentration (mol/dm³)

➤ Moles of solute per dm³ of solution.
➤ Concentration = Moles ÷ Volume

Calculate the concentration of 10 g of NaCl dissolved in 0.25 dm³.

➤ C = 10 ÷ 0.25 = 40 g/dm³

Convert 40 g/dm³ NaCl to mol/dm³ (Mr = 58.5).

➤ Moles = 40 ÷ 58.5 ≈ 0.684 mol/dm³

Molar Volume of Gas

➤ One mole of any gas occupies 24 dm³ at room temperature and pressure (RTP).

Volume of Gas (dm³)

Volume = Moles × 24

What is the volume of 2 mol of nitrogen gas at RTP?

➤ 2 × 24 = 48 dm³

How many moles of CO₂ are in 96 dm³ at RTP?

➤ 96 ÷ 24 = 4 mol