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# Lesson 3.3: Formula Weights

← Lesson 3.2: Lesson 3.4: →

## What does formula weight mean?

• The formula weight is the sum of the masses of the atoms or ions shown in a chemical formula.
• If the chemical formula is of a molecular compound like H2O, it will be the sum of the masses of the atoms.
• If the chemical formula is of an ionic compound like NaCl, it will be the sum of the masses of the ions.
• The somewhat awkward distinction exists because in the case of ionic substances the groups shown by the formula do not have independent existence.
• There are no independent NaCl molecules, but one Na+ ion for every one Cl- ion in all samples of NaCl.
• In practical terms we simply sum of the masses of what the formula represents making no distinction between the masses of atoms and ions

Atoms can be used to make molecules – groups of covalently bonded atoms. Think of a molecule like a bracelet and the atoms like the beads of the bracelet. Four atoms with their respective masses are shown below - all other atoms have masses that can be looked up on any periodic table. The unit u is a very small unit, called the atomic mass unit, and often written as amu. We have rounded the value given on the periodic table to 1 number after the decimal. For example, the periodic table gives the mass of carbon, C, as 12.011. We have rounded it to 12.0. The mass of the molecules is the sum of the masses of the atoms. The dashes indicate bonds between the atoms – what holds them together. ## Example 1: Find the mass of an ammonia molecule, NH3, in atomic mass units. NH3
Atom number mass
1 nitrogen atom = 1(14.0 u) = 14.0 u
3 hydrogen atoms = 3(1.0 u) = 3.0 u
= 17.0 u

## Example 2: Find the mass of a methane molecule, CH4, in atomic mass units. CH4
Atom number mass
1 carbon atom = 1(12.0 u) = 12.0 u
4 hydrogen atoms = 4(1.0 u) = 4.0 u
= 16.0 u

We find the formula weight of NaCl in the same way, but since there are no sodium chloride molecules, the meaning of the words is slightly different. We are finding the mass of the building blocks of socium chloride even when those blocks do not have independent existence.

 Since sodium chloride is ionic, there are no independent NaCl molecules. The solid is a group of ions and when the material melts or dissolves in water the ions separate from each other The purple spheres represent sodium ions, Na+, and the green spheres, chloride ions, Cl-  ## Example 3: Find the mass of a unit of sodium chloride, NaCl, in atomic mass units. NaCl
Atom number mass
1 sodium ion = 1(23.0 u) = 23.0 u
1 chloride ion = 1(35.4.0 u) = 35.4 u
= 58.4 u

We find the formula weight of Na2SO4 in the same way. However, it might seem more confusing because sulfate is a polyatomic ion - covalently bonded atoms, like a molecule, but one that has a charge and is therefore an ion. Since sodium sulfate is ionic, there are no independent Na2SO4 molecules. The solid is a group of ions and when the material melts or dissolves in water the ions are separated from each other. Sodium sulfate as a solid.  ## Example 4: Find the mass of a unit of sodium sulfate, Na2SO4, in atomic mass units. Na2SO4
Atom number mass
2 sodium ions = 2(23.0 u) = 46.0 u
1 sulfate ion = 1(32.1 u) + 4(16.0 u) = 96.1 u
= 142.1 u