How do you calculate the number of molecules in a substance?

• Find the amount (mass) of the substance in grams.
• Calculate the number of moles using the formula:

Number of moles = Mass of substance (g) ÷ Molar mass of substance (g/mol)

The molar mass is the mass of one mole of the substance, often found on the periodic table for elements or calculated from molecular formulas for compounds.

• Multiply the number of moles by Avogadro's number (6.022 × 10²³) to get the number of molecules:

Number of molecules = Number of moles × 6.022 × 10²³

Example:

If you have 18g of water (H₂O, molar mass = 18g/mol):

• Number of moles = 18g ÷ 18g/mol = 1 mol
• Number of molecules = 1 × 6.022 × 10²³ = 6.022 × 10²³ molecules

So, this method lets you calculate the number of molecules for any substance given its mass and molar mass.