How to calculate the number of moles

Calculating the number of moles

Question: How many moles are in 1000 g of sucrose?Mole calculations MOLES = MASS / MOLAR MASS

1. Obtain the atomic mass

First, write down the formula for sucrose: C12H22O11

Use the periodic table to obtain the atomic mass

2. Calculate the molar mass of sucrose

To calculate the molar mass of a substance, you must use the periodic table. Look for the atomic mass of each element present in the compound. Then use a table such as the one below to calculate the the sum of the atomic masses of the compound’s constituent atoms.

Element Symbol Atomic mass (amu) #n atoms  Mass
Hydrogen H 1.008 22 22(1.008) = 22.176 amu
Carbon C 12.01 12 12(12.01) = 144.12 amu
Oxygen O 16.00 11 11(16.00) = 176.00 amu
Molecular mass of sucrose

= (22.176 + 144.12 + 176.00)

= 342.296 amu

The molar mass of sucrose is simply the molecular mass expressed as g/mol.

Molar mass of sucrose = 342.296 g/mol

3. Calculate the number of moles

Apply the formula to calculate the number of moles

mass over molar mass = number of moles

The answer can be rounded to 3 mol.

(Note: The symbol for mole is “mol”)


Usefulness of moles

  • Knowing the number of moles allows you to calculate the number of atoms if you are dealing with chemical compounds.
  • Moles are very useful, they allow chemists to use chemical equations to determine the mass of reactants and products.

Read about Avogadro Constant

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