# How many molecules of C3H8 are required to react with 10 molecules of O2? C3H8(g) + 5 O2(g) ---> 3 CO2(g) + 4 H2O(l)

##### 1 Answer

#### Answer:

Two molecules of

#### Explanation:

The balanced equation is

This tells you that five molecules of

Ten molecules of

You might be able to do this problem in your head.

But it isn't as easy when you have more complicated numbers.

Then we use a **mole ratio** to set up the calculation.

The mole ratio is a fraction made from the coefficients in the balanced equation.

We use the mole ratio to convert the number of moles or molecules of one substance to the number of moles or molecules of another substance.

For example, in your equation

The mole ratio would be either

We would set up the equation as

Notice how we set up the molar ratio so that the units cancel to give us the correct units for the answer.

If we had used the other ratio, we would have gotten the wrong units.