Atoms and Molecules Formula Unit Mass

Formula Unit Mass

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula

unit of a compound.

Atoms and Molecules Formula Unit Mass

Formula Unit Mass

The method in which formula unit mass and molecular mass is calculated remains the same. The only difference remains that molecular mass is calculated for a molecule of any element, while formula unit mass is calculated for those compounds whose constituent elements are ions.

Atoms and Molecules Formula Unit Mass

Formula Unit Mass

Molecular mass is calculated for H2  while formula unit mass is calculated for

CaCl2.

Atoms and Molecules Formula Unit Mass

How to Find Formula Unit Mass Write a correct formula for the compound.

Look up the atomic mass of each element from the periodic table in the compound and round it to two decimal places.

Multiple the rounded atomic mass by the subscripts, if any.

Add all masses of elements together and use the unit, g/mol.

Atoms and Molecules Formula Unit Mass

Example

Find the formula mass of sodium chloride.

Formula: NaCl

Na = 23.00 (mass) X 1 (how many) = 23.00Cl = 35.45 (mass) X 1 (how many) = 35.45

Total = 58.45 g/mol

Thus, mass of one mole of sodium chloride = 58.45 grams

Atoms and Molecules Formula Unit Mass

Example

Find the formula mass of calcium phosphate.

Formula: Ca3 (PO4)2

Ca = 40.08 (mass) X 3 (how many) = 120.24P = 30.97 (mass) X 2 (how many) = 61.94

O = 16.00 (mass) X 8 (how many) = 128.00Total = 310.18 g/mol

Thus, mass of one mole of calcium phosphate = 310.18 grams

Atoms and Molecules Formula Unit Mass

Example

Calculate the formula mass of calcium chloride.

Formula: CaCl2

Ca = 40.08 (mass) X 1 (how many) = 40.08Cl = 35.45 (mass) X 2 (how many) = 70.90

Total = 110.98 g/mol

Thus, mass of one mole of calcium chloride = 110.98 grams