Relative Formula Mass/Relative Molecular Mass



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
Related Topics: More Chemistry Lessons

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.

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



Percent Mass of Elements

Using 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 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 in a compound.



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.






Calculating the Mass Percent of an Element 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.






Percent Mass of Water

Using 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

Calculating the Water of Crystallization








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



This video shows you how to calculate the percentage of water in the formula of a hydrated compound.
Compounds in this video CuSO4• 5H2O, CaCl2 • 2H2O, KAl(SO4)2•12H2O








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