Mole Ratios & Stoichiometry

Use a balanced equation as a recipe: coefficients give the mole ratio, molar mass converts grams to moles, and one ratio hop takes you from any substance to another.

Stoichiometry is recipe math for chemistry. The coefficients in a balanced equation are a mole ratio — they tell you how many moles of one substance you get from another. Convert grams to moles with the molar mass, hop across with the ratio, then convert back.

Balanced equation

2 H₂ + O₂ → 2 H₂O

Hydrogen and oxygen combine into water — two simpler substances forming one compound.

Given

Find

We will report how much H₂O is involved in moles and grams.

Worked solution

  1. 1. Start with moles of H₂

    1.000 mol of H₂

  2. 2. Apply the mole ratio

    The coefficients are the mole ratio. From the balanced equation, H₂O : H₂ = 2 : 2.

    mole ratio=22(H₂O per H₂)

    1.000 mol × (2 ÷ 2) = 1.000 mol H₂O

  3. 3. Moles of H₂O → grams

    Multiply by the molar mass of H₂O (18.015 g/mol):

    1.000 mol × 18.015 g/mol = 18.015 g

H₂O in moles

1.000

mol

H₂O in grams

18.015

g

The mole ratio comes straight from the balanced coefficients — it is the only step that crosses from one substance to another. Molar masses are summed from atomic masses, so the grams answers may carry a little rounding.

Balancing equations →·The mole →

For the reaction 2 H₂ + O₂ produces 2 H₂O: starting from 1.000 mol of H2, the mole ratio of H2O to H2 is 2 to 2, so you get 1.000 moles of H2O, which is 18.015 grams of H2O (molar mass 18.015 grams per mole).

Stoichiometry is recipe math

A balanced chemical equation is a recipe. A cookie recipe might say “2 cups flour make 1 dozen cookies”; the equation 2 H₂ + O₂ → 2 H₂O says “2 moles of hydrogen and 1 mole of oxygen make 2 moles of water.” Stoichiometry is just using those numbers to figure out how much — how much product you can make, or how much reactant you need. The catch: the recipe only works once the equation is balanced, because the coefficients are the amounts. The tool above runs every step for you, and it’s free to embed on your own site or LMS.

Coefficients are the mole ratio

The single most useful idea in this lesson: the coefficients are the mole ratio. They tell you the proportion in which substances react and form, counted in moles. From 2 H₂ + O₂ → 2 H₂O you can read off several ratios at once:

To “hop” from a known amount of one substance to an unknown amount of another, multiply by the matching ratio: moles of want = moles of have × (coefficient of want ÷ coefficient of have). Coefficients count particles, so the ratios are always in moles — never in grams.

Grams and moles: convert with molar mass

Lab measurements come in grams, but the ratio only speaks moles, so you translate with molar mass (grams per mole), summed from atomic masses on the periodic table. Going from grams to moles you divide by molar mass; going back from moles to grams you multiply. Building formulas and counting their atoms in the molecule builder is good practice for getting those molar masses right.

A worked example

How much water forms from 4.000 g of H₂ in 2 H₂ + O₂ → 2 H₂O?

  1. Grams → moles. The molar mass of H₂ is about 2.016 g/mol, so 4.000 g ÷ 2.016 g/mol = 1.984 mol H₂.
  2. Mole ratio. From the coefficients, H₂O : H₂ is 2 : 2 = 1, so 1.984 mol × (2 ÷ 2) = 1.984 mol H₂O.
  3. Moles → grams. The molar mass of H₂O is about 18.015 g/mol, so 1.984 mol × 18.015 g/mol ≈ 35.744 g H₂O.

Only step 2 crosses from one substance to another — that’s the mole ratio doing its job. Steps 1 and 3 are pure unit conversion within a single substance.

The road map to remember

Every grams-to-grams problem follows the same path:

grams (given) → moles (given) → moles (find) → grams (find)

Divide by molar mass to enter the world of moles, multiply by the mole ratio to move between substances, then multiply by molar mass to leave it again. Try different reactions and amounts in the tool above and watch the same three steps repeat every time.

Frequently asked questions

What is a mole ratio?
A mole ratio is the ratio between the amounts (in moles) of any two substances in a balanced equation, read directly from their coefficients. For 2 H₂ + O₂ → 2 H₂O, the mole ratio of water to hydrogen is 2 : 2, or 1 : 1, and the ratio of hydrogen to oxygen is 2 : 1.
Why does the equation have to be balanced first?
The coefficients are the mole ratio, and only a balanced equation has the correct coefficients. If the equation isn't balanced, the ratios are wrong and every amount you calculate from them will be wrong too.
How do I go from grams to grams?
Three steps. Convert the grams of the given substance to moles by dividing by its molar mass, multiply by the mole ratio from the coefficients to get moles of the substance you want, then multiply by that substance's molar mass to get grams. Grams → moles → ratio → moles → grams.
Do the coefficients work for moles or for grams?
Moles, not grams. Coefficients count particles, so they give ratios of moles (or molecules), never of mass. Mass isn't conserved as a simple ratio because different substances have different molar masses — that's exactly why you convert through moles.

Sources

Last reviewed: 2026-06-29

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