Question

one method for determining the identity of an unknown organic compound is through combustion analysis. a sample of an unknown substance is burned, and the amounts of water and carbon dioxide produced can be used to find the relative amounts of carbon, hydrogen, and oxygen. these devices can be modified to determine the composition of inorganic compounds as well. a combustion analysis shows that an unknown sample of gas is made of 85.5% carbon and 14.3% hydrogen. what is the empirical formula of the compound?
a ch
b ch₂
c c₂h
d ch₃

Explanation:

Step1: Assume 100g of the compound

Assume we have 100g of the compound. Then, the mass of carbon (\(m_{C}\)) is \(85.5g\) and the mass of hydrogen (\(m_{H}\)) is \(14.3g\).

Step2: Calculate the number of moles of each element

The molar mass of carbon (\(M_{C}\)) is \(12.01g/mol\), and the molar mass of hydrogen (\(M_{H}\)) is \(1.008g/mol\).
The number of moles of carbon (\(n_{C}\)) is \(n_{C}=\frac{m_{C}}{M_{C}}=\frac{85.5g}{12.01g/mol}\approx7.12mol\).
The number of moles of hydrogen (\(n_{H}\)) is \(n_{H}=\frac{m_{H}}{M_{H}}=\frac{14.3g}{1.008g/mol}\approx14.2mol\).

Step3: Find the mole - ratio

Divide each number of moles by the smaller number of moles. Here, \(n_{C}\approx7.12mol\) and \(n_{H}\approx14.2mol\).
\(\frac{n_{C}}{n_{C}} = 1\), \(\frac{n_{H}}{n_{C}}=\frac{14.2mol}{7.12mol}\approx2\).

Answer:

B. \(CH_{2}\)