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In a chemical reaction, compounds react in a set ratio. If the ratio is unbalanced, there will be leftover reactant. To understand this, you need to be familiar with the molar ratio or mole ratio.

### Mole Ratio Definition

A mole ratio is ​the ratio between the amounts in moles of any two compounds involved in a chemical reaction. Mole ratios are used as conversion factors between products and reactants in many chemistry problems. The mole ratio may be determined by examining the coefficients in front of formulas in a balanced chemical equation.

Also known as: The mole ratio is also called the mole-to-mole ratio.

### Mole Ratio Example: Balanced Equation

For the reaction:
2 H2(g) + O2(g) → 2 H2O(g)

The mole ratio between O2 and H2O is 1:2. For every 1 mole of O2 used, 2 moles of H2O are formed.

The mole ratio between H2 and H2O is 1:1. For every 2 moles of H2 used, 2 moles of H2O are formed. If 4 moles of hydrogen were used, then 4 moles of water would be produced.

### Unbalanced Equation Example

O3 → O2

By inspection, you can see this equation is not balanced because mass is not conserved. There are more oxygen atoms in ozone (O3) than there are in oxygen gas (O2). You cannot calculate mole ratio for an unbalanced equation. Balancing this equation yields:

2O3 → 3O2

Now you can use the coefficients in front of ozone and oxygen to find the mole ratio. The ratio is 2 ozone to 3 oxygen, or 2:3. How do you use this? Let's say you are asked to find how many grams of oxygen are produced when you react 0.2 grams of ozone.

1. The first step is to find how many moles of ozone are in 0.2 grams. (Remember, it's a molar ratio, so in most equations, the ratio is not the same for grams.)
2. To convert grams to moles, look up the atomic weight of oxygen on the periodic table. There are 16.00 grams of oxygen per mole.
3. To find how many moles there are in 0.2 grams, solve for:
x moles = 0.2 grams * (1 mole/16.00 grams).
You get 0.0125 moles.
4. Use the mole ratio to find how many moles of oxygen are produced by 0.0125 moles of ozone:
moles of oxygen = 0.0125 moles ozone * (3 moles oxygen/2 moles ozone).
Solving for this, you get 0.01875 moles of oxygen gas.
5. Finally, convert​ the number of moles of oxygen gas into grams for the answer:
grams of oxygen gas = 0.01875 moles * (16.00 grams/mole)
grams of oxygen gas = 0.3 grams

It should be fairly obvious that you could have plugged in the mole fraction right away in this particular example because only one type of atom was present on both sides of the equation. However, it's good to know the procedure for when you come across more complicated problems to solve.

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