SSCE VCE Chemistry Quantifying Atoms and Compounds: A sample of hydrocarbon contains

A sample of hydrocarbon contains 81.8% carbon by mass - VCE - SSCE Chemistry - Question 19 - 2003 - Paper 1

Question 19

A sample of hydrocarbon contains 81.8% carbon by mass. The empirical formula of the compound would be A. CH₂ B. CH₃ C. C₂H₅ D. C₃H₈

Worked Solution & Example Answer:A sample of hydrocarbon contains 81.8% carbon by mass - VCE - SSCE Chemistry - Question 19 - 2003 - Paper 1

Step 1

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Answer

Since the hydrocarbon contains 81.8% carbon by mass, we can deduce that the remaining mass must be hydrogen. Therefore, the sample contains:

Step 2

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Answer

To find the number of moles of each element, we use the molar masses:

Calculating the moles:

Moles of carbon = ( \frac{81.8 \text{ g}}{12.01 \text{ g/mol}} \approx 6.81 \text{ moles} )
Moles of hydrogen = ( \frac{18.2 \text{ g}}{1.008 \text{ g/mol}} \approx 18.06 \text{ moles} )

Step 3

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Next, we divide the number of moles of each by the smallest number of moles:

To simplify, we can round 2.65 to the nearest whole number (3), giving us a ratio of approximately:

Step 4

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Answer

From the simplest ratio of 1:3, the empirical formula of the compound is therefore: C₃H₈, which corresponds to option D.