How to calculate molar mass
- Type the formula with capital letters for each element: H2O, Ca(OH)2, K4[Fe(CN)6]. For a hydrate use a dot or star: CuSO4·5H2O or CuSO4*5H2O.
- Read the molar mass in g/mol and the table: atoms of each element, their atomic weight, their share of the mass and the mass percent.
- To convert grams to moles, type a mass in grams. You get the moles and the number of formula units (molecules).
Grams, moles and molar mass
The molar mass links the mass you can weigh to the number of particles. One mole is exactly 6.02214076 × 10²³ particles (the Avogadro constant), and the molar mass is the mass of one mole in grams.
- moles = mass in grams ÷ molar mass
- mass in grams = moles × molar mass
So 36 g of water is 36 ÷ 18.015 ≈ 2.0 mol, which is about 1.2 × 10²⁴ molecules.
Mass percent
The mass percent (percent composition) of an element is its share of the molar mass. In water, hydrogen is 2.016 ÷ 18.015 = 11.19% and oxygen 88.81%. It’s how you check an analysis result against a formula, or find how much of a compound is metal: copper is 25.45% of copper sulfate pentahydrate by mass.
Brackets and charges
Numbers after a bracket multiply everything inside it, so Ca(OH)₂ has 2 oxygen and 2 hydrogen atoms and (NH₄)₂SO₄ has 8 hydrogen atoms. Round, square and curly brackets all work. A charge such as SO4^2- or NH4+ is accepted and ignored: the electrons gained or lost weigh too little to change the answer at this precision.