AS-Level Chemistry Chemical Amounts Molar Masses Avogadro's Constant

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Chemical amounts

In chemistry, the amount of a substance is measured in moles. Chemists use the unit ‘mole’ to measure an amount of substance containing a standard number of atoms, molecules or ions. The word ‘mole’ entered the language of chemistry at the end of the nineteenth century. It is based on the Latin word for a heap or a pile. When chemists are determining formulae or working with equations, they need to measure amounts in moles. Chemists have balances to measure masses and graduated containers to measure volumes, but there is no instrument for measuring chemical amounts directly. Instead, chemists must first measure the masses or volumes of substances and then calculate the chemical amounts.

Molar masses

The key to working with chemical amounts in moles is to know the relative masses of different atoms. The accurate method for determining relative atomic masses involves the use of a mass spectrometer.

One mole of an element has a mass that is equal to its relative atomic mass in grams. So, the mass of one mole of carbon is 12.0 g and the mass of one mole of copper is 63.5 g. These masses of one mole are called molar masses (symbol M). So, the molar mass of carbon, M(C) = 12.0 g mol-1 and the molar mass of copper, M(Cu) = 63.5 g mol(-1).

Similarly the molar mass of the molecules of an element or a compound is numerically equal to its relative molecular mass. So, the molar mass of oxygen molecules, M(O2) = 32.0 g mol-1 and the molar mass of sulfuric acid, M(H2SO4) = 98.1 g mol-1.

Likewise, the molar mass of an ionic compound is numerically equal to its relative formula mass. The molar mass of magnesium nitrate, Mg(NO3)2, is therefore 148.3 g mol(-1).

Amount in moles

The mole is the SI unit for amount of substance. The name of the quantity is ‘mole’. Its unit is ‘mol’. So,
12 g of carbon contains 1 mol of carbon atoms
24 g of carbon contains 2 mol of carbon atoms
240 g of carbon contains 20 mol of carbon atoms.

Notice that:
amount of substance/mol = mass of substance/g molar mass/g mol–1
It is important to be precise about the chemical species involved when measuring amounts in moles. In calcium chloride, CaCl2, for example, there are two chloride ions, Cl–, combined with each calcium ion, Ca2+. So in one mole of calcium chloride there is one mole of calcium ions and two moles of chloride ions.

The Avogadro constant

Relative atomic masses show that one atom of carbon is 12 times heavier than one atom of hydrogen. This means that 12 g of carbon contains the same number of atoms as 1 g of hydrogen. Similarly, one atom of oxygen is 16 times as heavy as one atom of hydrogen, so 16 g of oxygen also contains the same number of atoms as 1 g of hydrogen.

In fact, the molar mass of every element (1 g of hydrogen, 12 g carbon, 16 g oxygen, and so on) contains the same number of atoms. This number is called the Avogadro constant, after the Italian scientist Amedeo Avogadro. Experiments show that the Avogadro constant, L, is 6.02 × 10(23) mol)=(–1). Written out in full this is 602 000 000 000 000 000 000 000 atoms, molecules or ions per mole.

The Avogadro constant is the number of atoms, molecules or formula units in one mole of any substance. So, one mole of oxygen (O2) contains 6.02 × 10(23) O2 molecules and two moles of oxygen (O2) contains 2 × 6.02 × 10(23) O2 molecules.

The number of atoms, molecules or formula units = amount of chemical/mol × the Avogadro constant/mol(–1)

Again, it is vital to specify the chemical species concerned in calculating the amount of a substance or the number of particles in a sample of a substance. For example, 2 g of hydrogen contains 2 mol of hydrogen (H) atoms (12.04 × 10(23) atoms) but only 1 mol of hydrogen (H2) molecules (6.02 × 10(23) molecules).