Amolecule of pentane is made of 5 carbon atoms and 12 hydrogen atoms. a collection of pentane molecules has 3.01 x 1024 carbon atoms. how
many hydrogen atoms are in the collection?

3.01 ă— 10^24 ă— (12/5) hydrogen atoms   
looking at the formula for the molecule, the ratio of carbon to hydrogen atoms is 5: 12, so if we divide the number of carbon atoms by 5 and then multiply by 12, we can find the number of hydrogen atoms. let's look at the available options and see what makes sense.   
3.01 ă— 10^24 ă— (12/5) hydrogen atoms 
* this is exactly correct.   
(3.01 ă— 10^24 / 5) hydrogen atoms 
* nope. this will tell you how many pentane molecules you have, but not the number of hydrogen atoms.   
3.01 ă— 10^24 ă— (5/12) hydrogen atoms 
* close, but the ratio (5/12) will tell you the number of carbon atoms you have if you give it the number of hydrogen atoms. so this choice is wrong.   
3.01 ă— 10^24 ă— 12 hydrogen atoms description 
* this would tell you the number of hydrogen atoms you have if you know the number of pentane molecules you have. so this choice is also wrong.

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