How to balance a chemical equation by hand
A balanced equation has the same number of each kind of atom on both sides. You can only change the numbers in front of formulas (coefficients), never the small numbers inside them.
- Write the skeleton. For example, Fe + O2 → Fe2O3.
- Start with the element in the fewest formulas, usually a metal. Leave hydrogen and oxygen for last, because they appear everywhere.
- Balance one element at a time. Oxygen is 2 on the left and 3 on the right, so the smallest common number is 6: 3O2 and 2Fe2O3.
- Fix what changed. 2Fe2O3 has 4 iron atoms, so you need 4Fe. Result: 4Fe + 3O2 → 2Fe2O3.
- Check and simplify. Count every element again and divide all coefficients by any common factor.
Redox and ionic equations
For reactions like KMnO4 with HCl, the trial method gets slow. JEE and NEET expect the ion-electron (half-reaction) method: split into oxidation and reduction halves, balance atoms other than O and H, then O with H2O, H with H+, and charge with electrons. Multiply the halves so the electrons cancel and add them.
This balancer solves the same equations exactly using algebra (it finds the smallest whole numbers that make every element and the total charge equal), so you can check your working.
Questions students ask
How do I type subscripts and charges?
Just type the numbers: H2SO4 for sulphuric acid. For ions, add ^ and the charge after the formula, like Fe^3+ or SO4^2-. Use e for an electron.
Why does it say my equation can be balanced in more than one way?
Some sets of reactants and products allow several independent reactions. Split it into separate reactions, or check that you haven't missed or added a product.
Can it balance redox equations?
Yes. Molecular redox equations like Cu + HNO3 and ionic ones like MnO4^- + Fe^2+ in acid both work, as long as you include H^+ and H2O where they belong.
Is balancing with fractions allowed?
In thermochemistry you'll sometimes see ½O2, but a standard balanced equation uses the smallest whole numbers. The balancer always gives whole numbers.