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Related Topics: More Chemistry Lessons

In these lessons, 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

*Example: *

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

*Solution: *

The formula for hydrogen gas is H_{2}. 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 H_{2}O. 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

*Example: *

What percentage of the mass of ammonium nitrate is nitrogen? (The formula for ammonium nitrate is NH_{4}NO_{3}, 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 NaNO_{3}? (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 in a compound.

Mass Percent Composition of an Element in a Compound

How do calculate the mass percent composition (or simply, the mass percent) of an element 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 massCalculating the Water of Crystallization.

This video outlines how to determine the percent, by mass, of water trapped in a hydrate's crystal lattice.

Compounds in this video CuSO

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