Relative masses

The actual mass of an atom is far too small to measure by weighing. So, the mass of an atom is compared relative to the mass of a carbon-12 atom - this is its relative mass. Relative masses have no units.

You need to know the following definitions:

1. Relative atomic mass (A_r_) - The weighted mean mass of an atom of an element, compared to 1/12^th of the mass of an atom of carbon-12.

For example, chlorine consists of two isotopes - ³⁵Cl (75% abundance) and ³⁷Cl (25% abundance). So its relative atomic mass is the weighted average of 35 and 37, which is 35.5.

1. Relative molecular mass (M_r_) is the average mass of a molecule compared to 1/12^th of the mass of an atom of carbon-12.

For example, a chlorine molecule (Cl_2_) consists of two atoms of chlorine each with a relative atomic mass of 35.5. So the relative molecular mass of Cl₂ is 2 x 35.5 = 71.0

1. Relative formula mass (M_r_) is the average mass of a formula unit compared to 1/12^th of the mass of an atom of carbon-12. Relative formula mass is used for ionic and giant covalent substances.

For example, the formula unit of sodium chloride (NaCl) consists of one sodium atom (A_r_ = 23.0) and one chlorine atom (A_r_ = 35.5). So the formula mass of NaCl is 23.0 + 35.5 = 58.5.

Calculating relative formula mass (M_r_)

The relative formula mass (M_r) of a compound is the sum of the relative atomic masses of all the atoms in the compound's formula unit. For molecular compounds comprised of covalently bonded molecules, the term relative molecular mass is used instead of relative formula mass.

The steps to calculate relative formula mass or relative formula mass are:

1. Write down the compound's formula unit or molecular formula.
2. Identify the number of atoms of each element present in the formula.
3. For each element, multiply the number of atoms by its relative atomic mass.
4. Sum up the values obtained in step 3.

Worked example 1 - Calculating M_r_

Calculate the relative molecular mass of water.

Step 1: Write down the molecular formula

H₂O

Step 2: Multiply each element's atom count by its A_r

H: 1.0 × 2 = 2.0

O: 16.0 × 1 = 16.0

Step 3: Calculate M_r

Relative molecular mass = 2.0 + 16.0 = 18.0

Worked example 2 - Calculating M_r_

Calculate the relative formula mass of magnesium oxide.

Step 1: Write down the formula unit

MgO

Step 2: Multiply each element's atom count by its A_r

Mg: 24.3 × 1 = 24.3

O: 16.0 × 1 = 16.0

Step 3: Calculate M_r

Relative formula mass = 24.3 + 16.0 = 40.3

The mole

The mole is a convenient unit that allows us to count extremely large numbers of atoms and molecules.

Avogadro's constant has a value of 6.02 x 10^23^. This very large number represents how many particles are contained in one mole.

The equation linking the number of moles, the number of particles, and Avogadro’s constant is:

$\text{number of moles }=\frac{\text{number of particles}}{\text{Avogadro's constant}}$

where particles refers to atoms, molecules, or ions, electrons.

Worked example 3 - Calculating number of moles

Calculate the number of moles in 2.71 x 10^24^ molecules of chlorine.

Avogadro's constant = 6.02 x 10²³ mol^-1

Step 1: Equation

$ \text{number of moles }=\frac{\text{number of particles}}{\text{Avogadro's constant}} $

Step 2: Substitution and correct evaluation

$ \text{number of moles }=\frac{(2.71\times10^{24})}{(6.02\times10^{23})}=4.50\text{ mol} $

Worked example 4 - Calculating number of atoms

Calculate the number of oxygen atoms in a 2.5 moles of oxygen atoms.

Avogadro's constant = 6.02 x 10²³ mol^-1

Step 1: Rearrange equation

Number of atoms = number of moles × Avogadro's constant

Step 2: Substitution and correct evaluation

Number of atoms = 2.5 × (6.02 × 10²³) = 1.5 × 10²⁴ atoms

Relating moles, mass, and molar mass

The molar mass (M) represents the mass in grams of 1 mole of a given substance.

• Molar mass is measured in units of grams per mole (g mol^-1).
• Molar mass is numerically equal to relative molecular/formula mass.

The equation linking the number of moles, mass in grams, and molar mass is:

$ \text{number of moles }=\frac{\text{mass in g}}{\text{molar mass}} $

Or:

$ n=\frac{m}{\text{M}} $

Where:

• n = number of moles (mol)
• m = mass (g)
• M = molar mass (g mol^-1^)

If molar mass (M) is be replaced by relative formula/molecuar mass (M_r_), the equation becomes:

$ n=\frac{m}{\text{M}_\text{r} }$

Worked example 5 - Calculating mass

Calculate the mass of 7.0 moles of magnesium oxide (MgO).

Step 1: Calculate M_r of MgO

Mg: 24.3 × 1 = 24.3

O: 16.0 × 1 = 16.0

M_r = 24.3 + 16.0 = 40.3 g mol^-1^

Step 2: Equation

$m ={n}\times{{M}_{r}}$

Step 3: Substitution and correct evaluation

m = 7.0 × 40.3 = 282.1 g

Worked example 6 - Calculating number of moles

Calculate the number of moles in 10.0 g of water (H₂O).

Step 1: Calculate M_r of H₂O

H: 1.0 × 2 = 2.0

O: 16.0 × 1 = 16.0

M_r = 2.0 + 16.0 = 18.0 g mol^-1^

Step 2: Equation

$n =\frac{m}{{M}_{r}}$

Step 3: Substitution and correct evaluation

$ \text{n }=\frac{10.0}{18.0}=0.56\text{ mol} $