How do you balance an equation with a polyatomic ion?

If a polyatomic ion such as sulfate (SO4) or nitrate (NO3) appears unchanged on both sides, balance it as a single unit instead of counting its atoms separately. It saves time and reduces mistakes.

What is a mole ratio?

Once an equation is balanced, the coefficients give the mole ratio — the ratio in which the substances react. In 2H2 + O2 -> 2H2O the mole ratio H2 : O2 : H2O is 2 : 1 : 2.

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Balancing chemical equations

Q: Why do chemical equations need to be balanced?

Because atoms are neither created nor destroyed in a chemical reaction (the law of conservation of mass). A balanced equation has the same number of each kind of atom on both sides, which also means the total mass is the same on both sides.

Q: What is the difference between a coefficient and a subscript?

A subscript is part of a formula and tells you how many atoms are in one molecule, like the 2 in H2O. A coefficient is the big number in front of a formula and tells you how many molecules react. You balance an equation by changing coefficients only, never subscripts.

Q: Which element should I balance first?

Start with an element that appears in only one compound on each side, and leave hydrogen and oxygen until last because they usually appear in several substances. Metals and then non-metals other than H and O is a common order.

A chemical equation is balanced when every element has the same number of atoms on the left as on the right. You balance it by putting whole-number coefficients in front of the formulas — never by changing a formula's subscripts. This page explains the method from the start, then gives you equations to balance yourself: you type the coefficients and it tells you which elements are still off and by how much, so you work it out rather than copy an answer.

Why equations have to balance

In a chemical reaction, atoms are only rearranged — none are created and none are destroyed. This is the law of conservation of mass. So if you start with, say, 6 oxygen atoms among the reactants, you must end with exactly 6 oxygen atoms among the products.

An unbalanced ("skeleton") equation like H2 + O2 → H2O shows the right substances but the wrong amounts: 2 oxygen atoms go in, only 1 comes out. Balancing fixes the amounts. It also makes the equation quantitative — the balanced coefficients are the mole ratio, which you need for any calculation about how much reacts or forms (that side of things uses the mole and molar mass).

Coefficient vs subscript

This is the rule people break most often.

A subscript is part of the formula. The 2 in H₂O means each water molecule has 2 hydrogen atoms. Change it and you change the substance — H₂O₂ is hydrogen peroxide, not water.
A coefficient is the number in front. 2H₂O means two whole water molecules — 4 H and 2 O in total.

You balance an equation by adjusting coefficients only.

The method

Check the formulas. Make sure every reactant and product formula is correct. A wrong formula can never be balanced.
Inventory the atoms. Count each element on the left and on the right.
Balance one element at a time. Start with an element that appears in just one substance on each side. Add a coefficient to make its counts match.
Recount after every change — one coefficient changes several counts.
Leave H and O until last, and balance a polyatomic ion (like SO₄) as a single unit if it's unchanged on both sides.
Reduce. Divide all coefficients by their greatest common factor.

Worked example — propane burning

Skeleton: C₃H₈ + O₂ → CO₂ + H₂O

Carbon appears once each side. Left has 3 C, right has 1. Put a 3 on CO₂: C₃H₈ + O₂ → 3CO₂ + H₂O

Hydrogen: left has 8, right has 2. Put a 4 on H₂O: C₃H₈ + O₂ → 3CO₂ + 4H₂O

Oxygen last. Right now has 3×2 + 4×1 = 10. Left has O₂, so put a 5 in front: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Recount — C: 3 = 3, H: 8 = 8, O: 10 = 10. Balanced, and the coefficients 1, 5, 3, 4 share no common factor. Done.

Practise it yourself

Type a coefficient in front of each substance, then press Check. The tool won't give you the answer — it tells you which elements are still unbalanced so you can fix them. Reveal the worked solution only if you're stuck.

— 0 balanced

Common mistakes

Changing a subscript. If you find yourself writing H₃O or O₃, stop — you can only put numbers in front.
Forgetting to recount. Adding a coefficient to fix one element usually unbalances another. Redo the whole inventory each time.
Balancing O or H too early. They turn up in many compounds, so their counts keep shifting. Save them for last.
Leaving it un-reduced. 4H₂ + 2O₂ → 4H₂O is "balanced" but not in lowest terms — divide through by 2.
Splitting a polyatomic ion. If NO₃ is unchanged on both sides, count it as one unit, not as N and 3 O.

Balancing equations — FAQ

Why do chemical equations need to be balanced?

Because atoms are neither created nor destroyed in a reaction — the law of conservation of mass. A balanced equation has the same count of every kind of atom on both sides, so the total mass is the same on both sides too.

What's the difference between a coefficient and a subscript?

A subscript is inside the formula (the 2 in H₂O — two H atoms per molecule). A coefficient is in front (2H₂O — two whole molecules). You balance with coefficients only; changing a subscript changes the substance.

Which element should I balance first?

One that appears in only one compound on each side. Do metals, then other non-metals, and leave hydrogen and oxygen until last because they're in many compounds.

How do I handle a polyatomic ion?

If it's unchanged on both sides (e.g. SO₄ stays as SO₄), balance it as one unit rather than counting its atoms. Only break it up if the ion is actually changed in the reaction.

What's a mole ratio and where does it come from?

It's the ratio of the balanced coefficients — the ratio in which the substances react. In 2H₂ + O₂ → 2H₂O the ratio H₂ : O₂ : H₂O is 2 : 1 : 2. You use it with molar mass for amount calculations.

Can every skeleton equation be balanced?

Only if the formulas are right and it represents a single real reaction. If it can't balance, a formula is usually wrong, or a substance is on the wrong side, or two reactions have been written as one.