A mole is an amount of substance: 6.02 × 10²³ particles (the Avogadro constant). The mass of one mole of a substance in grams equals its Mr: one mole of water has a mass of 18 g. Number of moles = mass ÷ Mr. In a balanced equation, the big numbers give the ratio of moles that react.
To find a reacting mass: convert the known mass to moles, use the equation's ratio, then convert back to mass. The limiting reactant is the one that is used up first; it decides how much product forms, and the other reactant is in excess. Concentration tells you how much solute is in a volume of solution: concentration (g/dm³) = mass ÷ volume (dm³), and 1 dm³ = 1000 cm³.
Worked example
How many moles are in 88 g of carbon dioxide (Mr 44)?
- moles = mass ÷ Mr.
- 88 ÷ 44 = 2.
Answer:There are 2 moles.
Key words
- mole
- 6.02 × 10²³ particles of a substance
- Avogadro constant
- 6.02 × 10²³ per mole
- limiting reactant
- the reactant that is used up first
- concentration
- amount of solute per volume of solution
New questions every time.
NGSS-referenced: HS-PS1-7