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Lesson 13 The Mole

  Calculating Molar Mass

The molar mass of elements is listed on the periodic table, but how do you determine the molar mass of compounds?



B13.7 A teacher pointing to the periodic table
It is simple to determine the molar mass of an element; it is listed on the periodic table.

For example, what is the molar mass (M) of sodium, Na?





MNa = 22.99 g/mol

For determining molar mass of an element, it is very important to recall the diatoms. Diatomic elements contain two atoms, so the molar mass is doubled.

For example, what is the molar mass of chlorine, Cl2?



MCl2 = 2 × 35.45 g/mol

MCl2 = 70.90 g/mol

To determine the molar mass of simple binary compounds in a 1:1 ratio, you just need to add the elemental molar masses together.

For example, what is the molar mass of sodium chloride, NaCl?

MNaCl = 22.99 g/mol + 35.45 g/mol

MNaCl = 58.44 g/mol

The following methodical approach will help you to calculate molar masses for more complex substances.

  1. Determine the molar mass of a substance to identify the number of atoms of each element.
  2. Multiply the number of atoms of each element by that atom’s molar mass.
  3. Add all of the values together to determine the molar mass of the compound.


Examples

Work through the following examples to gain an understanding of how to calculate molar mass. Each example has a video to go with it. To play the video, click on the play icon next to the example.

Determine the molar mass for water, H2O.

 
https://adlc.wistia.com/medias/2z48wtc4a3

Determine the molar mass for ethanol, C2H5OH.

 
https://adlc.wistia.com/medias/17illbdir6

Determine the molar mass for barium cyanide, Ba(CN)2.

 
https://adlc.wistia.com/medias/5vtx1owxx1

Determine the molar mass for iron(III) sulfate, Fe2(SO4)3.

 
https://adlc.wistia.com/medias/4wh46rave3


  Read This

Please read pages 108 and 109 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the steps for calculating molar mass. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice question to check your understanding of the concept you just learned. Make sure you write a complete answer to the practice question in your notes. After you have checked your answer, make corrections to your response (where necessary) to study from.

  1. Determine the molar mass for aluminium oxide, Al2O3.

  2. Determine the molar mass for calcium phosphate, Ca3(PO4)2.