Hydrocarbon stoichiometry question...?

Question

A sample of hydrocarbon contains 2.59 x 10^23 atoms of hydrogen and is 17.3% hydrogen by mass. If the molar mass of hydrocarbon is between 55 and 65 g/mol, how many mol of compound are present, and what is the mass of the sample?

I'm not asking you to give me the answer, I just want to know where to even start and just guide me on what to do? I've been working on this problem for ages and gotten no where. And also, do I take the average of the molar mass? (60 g/mol?)

Thanks in advance for any help! :)

Answer 1

how many moles of Hydrogen are in 2.59e23 atoms:

2.59e23 atoms @ 1 mole / 6.022e23 atoms = 0.430 moles of H

how massive is 0.430 moles of H @ 1.0079 g/mole = 0.433 grams of H

how massive is the sample if 17.3 % of it is 0.433 grams of H

0.433 g H @ 100% / 17.3 % = 2.5052 grams sample

your answer (3 sigfigs): 2.51 grams of sample

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to find the # of moles of compound, I need the molar mass.

which I can get from the molecular formula,

if i had the empirical formula & the approximate molar mass

find the empirical formula, by first finding the set of grams:

0.433 grams of H

2.51 - 0.433 grams H = 2.077 grams of carbon

find moles:

0.433 grams of H @ 1.01 g/mole = 0.430 moles H

2.077 grams of carbon @ 12.011g/mol = 0.173 moles C

ratio the moles:

0.430 moles H / 0.173 = 2.49 mole H

0.173 moles C / 0.173 = 1 mole C

double the ratio to have whole numbers & you have your empirical formula: C2 H5, whose empirical formula mass is 29 ,...

since the appx molar mass is between 55 & 65, the molecular formula is double the empirical formula: C4H10

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now going back to the first answer " 2.51 grams of sample" ,... & using the molar mass of C4H10  of  58.124 g/mol, your other question... "how many mol of compound are present"

2.51 grams sample @ 58.124 g/mole = 0.0432 moles of compound

your answer: 0.0432 moles of compound was present