Mini-Mods: General Chemistry - Moles

Created Feb. 7, 2024 by Paul Cooper

Molar Mass

One special property of the mole is the fact that it can also be related to an atom's or molecule's mass. One of the numbers displayed for each element on the periodic table is the atomic mass - how much each element weights in units of atomic mass units (amu). One amu is equal to approximately 1.66 x 10-24 g and is a convenient unit when dealing with individual atoms. But because we deal in chemistry in moles of atoms or molecules, we need a better unit - a unit that tells us the mass of one mole of a substance, or molar mass. Conveniently, the molar mass of an element is exactly the same number as its atomic mass - we just need to changed the units to grams/mol.    

 Let’s take a look at the element sodium (Na) as an example. A periodic table shows the atomic mass of sodium is 22.99 amu. But this also means the molar mass is 22.99 grams/mol. You should note that because molar mass contains one unit in the numerator and a different unit in the denominator, it is a conversion factor for converting between moles and mass.


Calculating Molar Masses

To determine the molar mass of an element (not including molecular elements), the atomic mass (in amu) can be read from a periodic table and simply changed to units of molar mass (g/mol).

To determine the molar mass of a compound, atomic masses are once again read from the periodic table and simply re-states with units of molar mass (g/mol), but the contribution from each element needs to be considered.

For example, in H2O there are two hydrogen atoms and one oxygen atom contributing to the overall mass of one water molecule. If we scaled that up to one mole of water, there are now two moles of hydrogen and one mole of oxygen contributing to the overall mass of this one mole. We therefore need to sum each contribution.

Molar mass of water = 2 x mass of hydrogen + 1 x mass of oxygen

Molar mass


Mole to Mass Calculations

What is the mass of 0.0358 mol of propane?

We are given mol and need to convert to mass - molar mass is a conversion factor for this process.

0.0358 \mathrm{\:mol\: C_{3}H_{8}}\: \times \frac{44.09\: \mathrm{g}}{1\: \mathrm{mol}}=1.58\:\mathrm{g}


Moles to Mass


Mass to Mole Calculations

How many moles of potassium sulfate are there in a sample of 0.246 g?

We are given mass and need to convert to mol. Once again, molar mass is a conversion factor for this.

0.246 \mathrm{\:g\: K_{2}SO_{4}}\: \times \frac{1\: \mathrm{mol}}{174.3\: \mathrm{g}}=0.00141\:\mathrm{g}


Mass to Moles


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