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)

The balanced equation is

#"C"_3"H"_8 + "5O"_2 → "3CO"_2 + "4H"_2"O"#

This tells you that five molecules of #"O"_2# require one molecule of #"C"_3"H"_8#, so

Ten molecules of #"O"_2# require twice as many molecules of #"C"_3"H"_8# or two molecules.

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 #("1 molecule C"_3"H"_8)/("5 molecules O"_2)# or #("5 molecules O"_2)/("1 molecule C"_3"H"_8)#.

We would set up the equation as

#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#

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.