The mass of one mole of atoms/molecules/ions is called its **molar mass ( M) **expressed in g/mol. To calculate the molar mass of an atom or a molecule, we are always going to need a periodic table. So, how do we find the molar mass in the periodic table?

For example, there are two numbers given for potassium in the periodic table: 19 and 39.098. The smaller one is the atomic number which shows the number of protons, and the larger one is the average atomic mass:

What’s important for today’s topic is that **numerically, the molar mass is equal to the average atomic mass **of the given atom, so we can look it up in the periodic table. For potassium then, it is 39.098 g/mol which means one mole of potassium atoms weighs 39.098 g. And we can do this for all the elements in the periodic table.

If there is a subscript in the symbol, then you need to multiply the molar mass by that number. For example, to find the molar of P_{4}, we multiply 30.974 g/mol by 4:

M (P_{4}) = 30.974 g/mol x 4 = 123.896 g/mol

Usually, we round off the molar mass to 1 or 2 decimal places, so for P, we’d use 123.9 or 123.90 for its molar mass.

**The Molar Mass of Molecules**

To calculate the molar mass of a molecule, we add the molar mass of each constituent atom by the corresponding subscript.

For example, the molar mass of water would be:

M (H_{2}O) = 2 x 1.0 + 16.0 = 18.0 g/mol

If the formula of the molecule is not given, you will need to first determine it.

**For example, **determine the molar mass of phosphorous pentachloride.

** **The formula of phosphorous pentachloride is PCl5. Go over naming covalent compounds to refresh some of the concepts here.

To calculate the molar mass of PCl_{5}, we multiply the molar mass of each element by its subscript:

** **

M (PCl_{5}) = M (P) + 5x M (Cl)

M (PCl_{5}) = 30.1 + 5 x 35.5 = 207.6 g/mol

** **

**Another example:** what is the molar mass of aluminum sulfate?

First, we know that the aluminum ion has a 3+ charge. You need to know the common polyatomic ions from where we find that sulfate is the SO_{4}^{2-} ion. Next, we write the ions next to each other and add the correct subscripts:

And now, we multiply the molar mass of each element by its subscript. Remember, the number outside of parenthesis applies to all the elements in it.

M (Al_{2}(SO_{4})_{3}) = 2 x M (Al) + 3 x M (S) + 3 x 4 x M (O)

M (Al_{2}(SO_{4})_{3}) = 2 x 27.0 + 3 x 32.1 + 3 x 4 x 16.0 = 342.3 g/mol

**The Molar Mass of Ions**

Do not worry about the charge of the ion. It is a result of either losing or gaining an electron(s) which has negligible mass compared to atoms and molecules.

**For example**, what is the molar mass of the nitrate ion?

The formula of nitrate ion is NO_{3}^{–}, therefore, the molar mass would be:

M (NO_{3}^{–}) = M (N) + 3 x M (O) = 14.0 + 3 x 16.0 = 62.0 g/mol

**The Molar Mass of Hydrates**

To calculate the molar mass of a hydrate, determine the molar mass of the salt and add to the molar mass of the water multiplied by its subscript.

**For example**, calculate the molar mass of nickel (II) chloride hexahydrate.

He formula of the hydrate is NiCl_{2}·6H_{2}O, and therefore, the molar mass would be:

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M (NiCl_{2}·6H_{2}O) = M (Ni) + 2 M (Cl) + 6 M (H_{2}O)

M = 58.7 + 2 x 35.5 + 6 x 18.0 = 237.7 g/mol

Calculating the molar mass is essential as it lays the foundation for most of the calculations in stoichiometry such as the mass, moles, number of particles and etc.

**Check Also**

- The Mole and Molar Mass
- How To Convert Grams To Moles
- Grams to Molecules and Molecules to Mass
- How To Convert Grams To Number of Atoms
**Mass, Moles, and Number of Particles Practice**- Percent Composition and Empirical Formula
- Percent Composition and Empirical Formula Practice Problems
- Stoichiometry of Chemical Reactions
- Limiting Reactant
- How To Find The Amount of Excess Reactant
**Limiting Reactant Practice Problems**- Reaction/Percent Yield
**Stoichiometry Practice Problems**