I've seen other questions like this on the platform answered, so I'll try to break each step down a little further.

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First, find the molecular weight of C_{3}H_{8 }from the periodic table:

MW Carbon (C) = 12.01g/mol

MW Hydrogen (H) = 1.01 g/mol

3 C = 12.01 g/mol x 3 moles = 36.03 grams

8 H = 1.01 g/mol x 8 moles = 8.08 grams

Add the masses of the carbon and hydrogen together

36.03 grams C

+ 8.08 grams H

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**44.11 grams/mole is the molecular weight of C**_{3}**H**_{8}

So the number of moles in 73.7 grams of C_{3}H_{8 }is calculated like so:

73.3 grams C_{3}H_{8 }× __1 mole C___{3}__H___{8 }=** 1.66 moles C**_{3}**H**_{8}

44.11 grams

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In order to know how much oxygen it takes to completely react with/burn up 1.66 moles C_{3}H_{8}, we need to convert the moles of C_{3}H_{8 }to moles of O_{2}.The only way we can do this is through the balanced equation since it's the only way to relate the two molecules.

From the balanced equation in the problem, we see on the left side that there are 5 O_{2} for every 1 C_{3}H_{8}. You could say they are in a 5:1 ratio. (5 mole O_{2} : 1 mole C_{3}H_{8})

We have 1.66 moles C_{3}H_{8 }so:

1.66 moles C_{3}H_{8 }x __5 moles O___{2 }= 8.3 moles O_{2}

1 mole C_{3}H_{8}

This means that it takes **8.3 moles of O**_{2}** to completely burn up the 1.66 moles of C**_{3}**H**_{8.}

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Now we can look at what the question is actually asking us: How many **grams** of O_{2} does it take to burn up the C_{3}H_{8}?

Now we can use the molecular weight of O_{2} to convert moles of O_{2 }to grams of O_{2}. This is somewhat the reverse of what we did in the first part of the problem with C_{3}H_{8}.

First, we calculate the molecular weight of O_{2}.

MW Oxygen (O) = 15.9999 g/mole

2 O = 15.9999 g/mol x 2 moles = 31.9998 grams

We only have one element, so the **molecular weight of O**_{2}** is 31.99 grams/mole**

So we use this to convert the moles of oxygen to the grams of oxygen that the problem requires.

8.3 moles O_{2} x __31.99 grams__ = 265.52 grams O_{2}

** ** 1 mole O_{2}

**So it takes 265.52 grams of O**_{2}** to completely burn up 73.7 grams (aka 1.66 moles) of C**_{3}**H**_{8}**!**

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Hope this helps!