Topic 3: Quantitative Chemistry

What does the law of conservation of mass state?

No atoms are lost or made during a chemical reaction, so mass of products = mass of reactants.

What is relative formula mass (Mr)?

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

Why might mass appear to change in a reaction?

A gas may enter from or escape into the atmosphere and not be measured.

What causes uncertainty in chemical measurements?

All measurements have a degree of uncertainty, requiring estimation and careful calculation.

What is a mole?

The amount of substance that contains the same number of particles as 12g of carbon-12.

What is the symbol for mole and the Avogadro constant?

Symbol: mol; Avogadro constant: 6.02 × 10²³ particles per mole.

What is the formula linking mass, moles, and molar mass?

Mass = Moles × Molar mass (Mr)

How many moles are in 42g of carbon (Mr = 12)?

42 ÷ 12 = 3.5 moles

What does a balanced symbol equation tell you about moles?

The ratios of moles of reactants and products.

How do you calculate the mass of products/reactants from a balanced equation?

1. Convert known masses to moles

2. Use ratios from equation

3. Convert moles back to mass

What is a limiting reactant?

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

How do you identify the limiting reactant?

Calculate moles of each reactant and compare using the balanced equation.

What is the formula for concentration of a solution?

Concentration = Mass ÷ Volume (g/dm³)

How does changing mass or volume affect concentration?

• More solute or less volume → higher concentration

• Less solute or more volume → lower concentration