# 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)

Jun 4, 2014

Two molecules of ${\text{C"_3"H}}_{8}$ are required.

#### Explanation:

The balanced equation is

$\text{C"_3"H"_8 + "5O"_2 → "3CO"_2 + "4H"_2"O}$

This tells you that five molecules of ${\text{O}}_{2}$ require one molecule of ${\text{C"_3"H}}_{8}$, so

Ten molecules of ${\text{O}}_{2}$ require twice as many molecules of ${\text{C"_3"H}}_{8}$ or two molecules.

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.

The mole ratio would be either $\left({\text{1 molecule C"_3"H"_8)/("5 molecules O}}_{2}\right)$ or $\left({\text{5 molecules O"_2)/("1 molecule C"_3"H}}_{8}\right)$.
10color(red)(cancel(color(black)("molecules O"_2))) × ("1 molecule C"_3"H"_8)/(5color(red)(cancel(color(black)("molecules O"_2)))) = "2 molecules C"_3"H"_8