The core idea
Add the atomic masses of every atom in a chemical formula, accounting for subscripts and groups. For H₂O: 2 × 1.008 + 15.999 = 18.015 g/mol. Hydrates must include their waters of crystallization.
Try it yourself
Open the Molar Mass Calculator and enter your own values. Keep units consistent and compare the answer with the worked explanation.
Check your work
Write down your known quantities, convert units before substitution and confirm that your final units match the quantity you intended to calculate.