Question

Given a hydrocarbon containing 7.7% by mass of hydrogen and 92.3% by mass of carbon, with a relative molar mass of 78:

(i) Derive the empirical formula of the compound and hence its molecular formula.
(ii) Name the hydrocarbon and write its structural formula. (H=1;C=12)

What is a functional group?

Name the functional group present in the following homologous series:
(i) Alkanols
(ii) Alkanoic acid

State 3 characteristics of a homologous series.

Answer 1

The empirical formula of the hydrocarbon with 7.7% hydrogen and 92.3% carbon is CH, and its molecular formula is C6H6, which is benzene. The functional group present in alkanols is the hydroxyl group (-OH), and in alkanoic acids, it is the carboxyl group (-COOH). Three characteristics of a homologous series are the same general formula, gradual change in physical and chemical properties, and gradual increase in boiling point and molecular weight.

Step-by-step explanation:

Given a hydrocarbon containing 7.7% by mass of hydrogen and 92.3% by mass of carbon, we can calculate the empirical formula of the compound. Assuming 100g of the compound, we have 7.7g of hydrogen and 92.3g of carbon. Converting the mass of each element into moles, we find that there are approximately 7.61 moles of hydrogen and 7.68 moles of carbon. Dividing these numbers by the smallest mole value, we get a ratio of approximately 1:1. Therefore, the empirical formula is CH.

To determine the molecular formula, we need the relative molar mass. The relative molar mass of the compound is given as 78. The molecular formula is a multiple of the empirical formula, so we need to find the factor that, when multiplied by CH, gives the relative molar mass of 78. Since the empirical formula CH has a molar mass of 13 (12+1), the molecular formula will be CH multiplied by 6, resulting in C6H6. This is the molecular formula of benzene.

A functional group is a specific arrangement of atoms or a group of atoms within a molecule that is responsible for the characteristic chemical reactions of that compound. In the homologous series of alkanols, the functional group is the hydroxyl group (-OH). In the homologous series of alkanoic acids, the functional group is the carboxyl group (-COOH).

Three characteristics of a homologous series are:

1. They have the same general formula.
2. They exhibit a gradual change in physical and chemical properties as the number of carbon atoms increases.
3. They show a gradual increase in boiling point and molecular weight as the number of carbon atoms increases.