# Relative Formula Mass / Relative Molecular Mass

Related Topics: More Chemistry Lessons

In this lesson, we will learn
• how to calculate the relative formula mass or relative molecular mass
• how to use the relative molecular mass to calculate the percent mass of an element in a compound
• how to use the relative molecular mass to calculate the percent mass of water in a compound

The following diagram shows how to calculate the relative molecular mass or relative formula mass. Scroll down the page for more examples and solutions.

What is relative formula mass and 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 Mr.
If the substance is made of simple molecules, this mass may also be called the relative molecular mass.

How to calculate the relative formula 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

How to find the Percent Mass of Elements in a compound?
How to use Mr to calculate the percent 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 nitrogen in the formula = 28
Mass of nitrogen as a fraction of the total =
Mass of nitrogen 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

How to find percent by mass and percent composition?
Example:
Find the percent composition by mass of potassium dichromate(K2Cr2O7)
Step 1: Find the molar mass of the compound.
Step 2: Divide the total mass of each element by the molar mass and multiply by 100 to find the % mass. How find the percent of an element in a compound?
Finding the percent by mass means finding the mass of the elements in the compound and adding the masses for the total mass.
Example:
You have a 7364 milligrams sample of SO2. How many grams of sulfur are in the sample?

How to calculate the Mass Percent of an Element in a Compound?
Mass Percent Composition of an Element in a Compound
To calculate the mass percent composition (or simply, the mass percent) of an element in a compound, we divide the mass of the element in 1 mol of the compound with the mass of 1 mol of the compound and multiply by 100%.
Examples:
1. What is the mass percent of carbon in carbon dioxide?
2. What is the mass percent of oxygen in carbon dioxide? How to find the Percent Mass of Water in a compound?
How to use Mr to calculate the percent 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 =
Mass of hydrogen as percentage of total mass

How to calculate the Water of Crystallization?
This video outlines how to determine the percent, by mass, of water trapped in a hydrate's crystal lattice.
Example:
What is the percent of water, by mass, in the hydrate CuSO4•2H2O How to calculate the percentage of water in the formula of a hydrated compound?
Example:
Determine the % water in the following hydrates:
a) CuSO4•5H2O
b) CaCl2•2H2O
c) KAl(SO4)2•12H2O

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