Foundations · Learning guide

How to read a chemical formula

A formula is a compact inventory of the atoms in a substance. Reading it correctly is the first step toward calculating quantities.

Symbols and subscripts

An element symbol starts with a capital letter and may include a lowercase letter. Co means cobalt; CO means carbon and oxygen. In H2O, the subscript 2 belongs to hydrogen only. An omitted subscript means one atom.

Parentheses multiply a group

Calcium hydroxide, Ca(OH)2, contains one calcium, two oxygen and two hydrogen atoms. For Al2(SO4)3, multiply everything inside the parentheses by three: two aluminium, three sulfur and twelve oxygen atoms.

From formula to mass

Multiply each atom count by its standard atomic weight, then add the contributions. Water has an approximate molar mass of 18.015 g/mol. These rounded values are suitable for learning, not precision laboratory certification.

Try it

Compare CO and CO2 in the calculator. The difference is approximately one oxygen atomic weight. This checks both your interpretation and your arithmetic.

Further reading: Reference source ↗. Written for this starter collection. Check primary references for specialised use.