Question

A 27.0-gram sample of an unknown hydrocarbon was burned in excess oxygen to form 88.0 grams of carbon dioxide and 27.0 grams of water. What is a possible molecular formula of the hydrocarbon?

Answer 1

Answer:

$C_4H_6$

Explanation:

$Moles =Given\ mass \times {Molar\ mass}$

Mass of water obtained = 27.0 g

Moles of $H_2O$ = 27.0 / 18 = 1.5 moles

2 moles of hydrogen atoms are present in 1 mole of water. So,

Moles of H = 2 x 1.5 = 3 moles

Molar mass of H atom = 1.008 g/mol

Mass of H in molecule = 3 x 1.008 = 3.024

Given that the compound is hydrocarbon. So it only contains hydrogen and carbon. So,

Mass of C in the sample = Total mass - Mass of H

Mass of the sample = 27 g

Mass of C in sample = 27 - 3.024 = 23.976 g  

Molar mass of  C = 12 g/mol

Moles of C  = 23.976  / 12 = 1.998 moles

Taking the simplest ratio for H and C as:

3.024 : 1.998 = 3 : 2

The empirical formula is = $C_2H_3$

Molecular formulas is the actual number of atoms of each element in the compound while empirical formulas is the simplest or reduced ratio of the elements in the compound.

Thus,  

Molecular mass = n × Empirical mass

Where, n is any positive number from 1, 2, 3...

Since, the hydrogen is present in odd number in the empirical formula.So, possible molecular formula is:

Molecular mass = 2 × Empirical mass

The least possible formula of hydrocarbon = $C_4H_6$