How to calculate molar mass step by step
Read a chemical formula, count atoms through parentheses and hydrates, and sum atomic weights, with worked H2SO4 and CuSO4.5H2O examples.
Reading a chemical formula
A formula is a compact tally of atoms. A capital letter starts each element symbol and any lowercase letters finish it, so Na is one element while NaCl is two. A number after a symbol is a subscript that counts only the atom or group immediately before it. H2SO4 therefore means two hydrogen atoms, one sulfur atom and four oxygen atoms, not twenty four of anything.
Counting atoms in groups and hydrates
Parentheses and brackets let a subscript apply to a whole group. In Ca(OH)2 the 2 doubles both the oxygen and the hydrogen inside the brackets, giving one calcium, two oxygen and two hydrogen. A dot marks a hydrate, so CuSO4.5H2O attaches five separate water molecules to the copper sulfate. The five multiplies everything after the dot, adding ten hydrogen atoms and five more oxygen atoms to the count.
Summing the atomic weights
Once you know how many of each atom you have, multiply each count by that element's standard atomic weight and add the products. For H2SO4 that is two times 1.008 for hydrogen, one times 32.06 for sulfur, and four times 15.999 for oxygen. The subtotals are 2.016, 32.06 and 63.996, which sum to a molar mass of 98.072 g/mol. The breakdown table on this page lays out exactly these three lines.
A hydrate worked in full
Copper sulfate pentahydrate, CuSO4.5H2O, shows every rule at once. Copper contributes 63.546 and sulfur 32.06. Oxygen appears four times in the sulfate and five more times in the water for nine atoms total, contributing 143.991. Hydrogen appears ten times across the five waters, contributing 10.08. Adding those gives 249.677 g/mol, and the tool orders the breakdown by first appearance: Cu, S, O, then H.