Molecular Mass Calculations

The molecular mass of a molecule is the total mass of all the atoms making up the molecule. This example problem illustrates how to find the molecular mass of a compound or molecule.

Molecular Mass Problem

Find the molecular mass of table sugar (sucrose), which has a molecular formula C₁₂H₂₂O₁₁.

Solution

To find the molecular mass, add the atomic masses of all of the atoms in the molecule. Find the atomic mass for each element by using the mass given in the Periodic Table. Multiply the subscript (number of atoms) times the atomic mass of that element and add the masses of all of the elements in the molecule to get the molecular mass. For example, multiple the subscript 12 times the atomic mass of carbon (C). It helps to know the symbols for the elements if you don't know them already.

If you round off the atomic masses to four significant figures, you get:

molecular mass C₁₂H₂₂O₁₁ = 12(mass of C) + 22(mass of H) + 11(mass of O)
molecular mass C₁₂H₂₂O₁₁ = 12(12.01) + 22(1.008) + 11(16.00)
molecular mass C₁₂H₂₂O₁₁ = = 342.30

Answer

342.30

Note that a sugar molecule is about 19 times heavier than a water molecule!

When performing the calculation, watch your significant figures. It's common to work a problem correctly, yet get the wrong answer because it's not reported using the correct number of digits. Close counts in real life, but it's not helpful if you're working chemistry problems for a class.

For more practice, download or print these worksheets:

• Formula or Molar Mass Worksheet (pdf)
• Formula or Molar Mass Worksheet Answers (pdf)

Note About Molecular Mass and Isotopes

The molecular mass calculations made using the atomic masses on the periodic table apply for general calculations, but aren't accurate when known isotopes of atoms are present in a compound. This is because the periodic table lists values that are a weighted average of the mass of all natural isotopes of each element. If you are performing calculations using a molecule that contains a specific isotope, use its mass value. This will be the sum of the masses of its protons and neutrons. For example, if all the hydrogen atoms in a molecule are replaced by deuterium, the mass for hydrogen would be 2.000, not 1.008.

Problem

Find the molecular mass of glucose, which has a molecular formula C6H12O6.

Solution

To find the molecular mass, add the atomic masses of all of the atoms in the molecule. Find the atomic mass for each element by using the mass given in the Periodic Table. Multiply the subscript (number of atoms) times the atomic mass of that element and add the masses of all of the elements in the molecule to get the molecular mass. If we round off the atomic masses to four significant figures, we get:

molecular mass C6H12O6 = 6(12.01) + 12(1.008) + 6(16.00) =180.16

Answer

180.16

For more practice, download or print these worksheets:

• Formula or Molar Mass Worksheet (pdf)
• Formula or Molas Mass Worksheet Answers (pdf)