Relative Formula Mass / Relative Molecular Mass

• The relative formula mass of a substance is the sum of the relative atomic masses of the elements present in a formula unit.
• The symbol for relative formula mass is M_r
• If the substance is made of simple molecules, this mass may also be called the relative molecular mass.

Example:

What is the relative mass formula of hydrogen gas? (Relative atomic mass: H = 1)

Solution:

The formula for hydrogen gas is H₂. Each molecule contains 2 hydrogen atoms.

The relative mass formula of hydrogen gas is

Example:

What is the relative mass formula of water? (Relative atomic masses: H = 1, O = 16)

Solution:

The formula for water is H₂O. Each molecule contains 2 hydrogen atoms and 1 oxygen atom.

The relative mass formula of water is

Example:

What is the relative mass formula of sodium chloride? (Relative atomic masses: Na = 23, Cl = 35.5)

Solution:

Sodium Chloride is an ionic solid with the formula Na⁺Cl^-.

The relative mass formula of sodium chloride is

• Using M_r to calculate the percentage mass of an element in a compound

Example:

What percentage of the mass of ammonium nitrate is nitrogen? (The formula for ammonium nitrate is NH₄NO₃, Relative atomic masses: H = 1, O = 16, N = 14)

Solution:

Mass of hydrogen in the formula = 28

Mass of hydrogen as a fraction of the total =

Mass of hydrogen as percentage of total mass

Example:

What percentage of the mass of O in NaNO₃? (Relative atomic masses: Na = 23, O = 16, N = 14)

Solution:

Mass of O in the formula = 48

Mass of O as a fraction of the total =

Mass of O as percentage of total mass

The following video gives an example of calculating percent mass of elements.

• Using M_r to calculate the percentage mass of water in a compound

Example:

What percentage of the mass of magnesium sulfate is water? (Given that the formula for magnesium sulfate is , Relative atomic masses: H = 1, O = 16, S = 32, Mg = 24)

Solution:

Mass of water in the formula =

Mass of water as a fraction of the total =

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