Chemistry Topic 3: Chemical Equations - Moles and Mole Ratio

A mole is a unit for measuring the amount of a substance.

Example: 1 dozen = 12
Example: 1 pair = 2
Therefore, 1 mole = 6.02 imes 10^{23} (which is 602,000,000,000,000,000,000,000).

Facts About Moles

  • A mole of pennies would cover the Earth to a depth of 400 meters (about 5 city blocks).

  • A mole of hockey pucks would weigh the same as the Moon.

  • If you spent a mole of dollar bills at a rate of one million dollars per second (1,000,000/second) starting 13 billion years ago, you would still have 19 billion trillion dollars left.

  • A mole of sugar weighs about 300g (similar to a can of soup).

What is the Number 6.02 imes 10^{23}?
The mole is the amount of stuff that has the same number of particles as 12g of carbon-12 (the most common carbon). This number is called Avogadro's number: 6.02 imes 10^{23}.

Molar Mass of an Element
Molar mass is the mass in grams of 1 mole of a substance. You can find this on the periodic table. Units are g/mol.

Molar Mass of a Compound
To find the molar mass of a molecule, add up the molar masses of each atom in the formula.

Example: NaCl

  • Na: 22.99 g/mol

  • Cl: 35.45 g/mol

  • Total: 58.44 g/mol

Example: CaCl2

  • Ca: 40.1 g/mol

  • Cl: 2 x 35.5 g/mol = 71.0 g

  • Total: 111.1 g

Practice Questions - Moles

  1. Calculate the molar mass of:

    • FeSO_4

    • MgSO_4

    • Al(OH)_3

    • C{12}H{22}O_{11}

Examples for Determining Moles

  • Formula: n = \frac{m}{M}

    • Here, n is the number of moles, m is mass, and M is molar mass.

    • Mass is NOT the same as molar mass! You have to find molar mass first.

Mole Ratios
Mole ratios tell us how many molecules to mix in a reaction, based on the coefficients in a balanced equation.