The mole and Avogadro's constant - Cambridge OCR GatewayMoles and masses - Higher
What is the mole and Avogadro's constant? The mole is the unit for amount of substance. The number of particles in a substance can be found using the Avogadro constant. The mass of product depends upon the mass of limiting reactant.
The molar mass of a substance is the mass of one moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 ×10²³ number of particles). of that substance. It is equal to:
the relative atomic massThe mean relative mass of the atoms of the different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom. or Ar in grams for atomThe smallest part of an element that can exist. and metalShiny element that is a good conductor of electricity and heat, and which forms basic oxides.
the relative formula massThe sum of the relative atomic masses of the atoms in a chemical formula. or Mr in grams for moleculeA collection of two or more atoms held together by chemical bonds. and compoundA substance formed by the chemical union of two or more elements.
The table shows some examples.
Substance
Formula
Ar or Mr
Molar mass (g/mol)
Neon
Ne
Ar = 20.2
20.2
Magnesium
Mg
Ar = 24.3
24.3
Oxygen
O2
Mr = 2 × 16.0 = 32.0
32.0
Calcium carbonate
CaCO3
Mr = 40.1 + 12.0 + (3 × 16.0) = 100.1
100.1
Substance
Neon
Formula
Ne
Ar or Mr
Ar = 20.2
Molar mass (g/mol)
20.2
Substance
Magnesium
Formula
Mg
Ar or Mr
Ar = 24.3
Molar mass (g/mol)
24.3
Substance
Oxygen
Formula
O2
Ar or Mr
Mr = 2 × 16.0 = 32.0
Molar mass (g/mol)
32.0
Substance
Calcium carbonate
Formula
CaCO3
Ar or Mr
Mr = 40.1 + 12.0 + (3 × 16.0) = 100.1
Molar mass (g/mol)
100.1
Masses from moles
The massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of a substance in grams can be calculated from its amount in moles and its molar mass:
mass (g) = molar mass (g/mol) × amount (mol)
Example
Calculate the mass of 0.25 mol of carbon dioxide molecules, CO2. (Relative atomic masses: C = 12.0, O = 16.0)
