The melting points of three members A, B and C of a homologous series of hydrocarbons are -183°C, -138°C and, -130°C, respectively. Which one of the three would have the lowest number of carbon atoms in its molecule ?

It is known that as the number of carbon atoms in the molecules of hydrocarbons (like alkanes) increases, their melting points and boiling points also go on increasing.

So, the hydrocarbon having the lowest melting point (or boiling point) will have the lowest number of carbon atoms in its molecule (and the hydrocarbon having the highest melting point or boiling point will have the maximum number of carbon atoms in its molecule).

In this case, out of the three hydrocarbons A, B and C, the hydrocarbon A has the lowest melting point of -183°C (minus one - eighty - three degrees celsius), therefore, hydrocarbon A would have the lowest number of carbon atoms (or minimum number of carbon atoms) in its molecule.

That is, hydrocarbon A will have the lowest molecular mass. (Please note that the temperature, -183°C is lower than, -138°C as well as, -130°C. This is because these temperatures are in 'minus' values).

The boiling points of three saturated hydrocarbons P, Q and R are -162°C, -42.2°C and, -0.5°C respectively. Which of these hydrocarbons would have the maximum number of carbon atoms in its molecule ?

The hydrocarbon having the highest boiling point will have the maximum number of carbon atoms in its molecule.

Here, out of the three hydrocarbons P, Q and R, the hydrocarbon R has the highest boiling point of -0.5^oC, therefore, hydrocarbon R would have the maximum number of carbon atoms in its molecule. It will also have the maximum molecular mass.

Two alkanes 'A' and 'B' have 3 and 5 carbon atoms in their molecules, respectively. In which physical state will they occur at the room temperature ?

It is known that alkanes having 1 to 4 carbon atoms per molecule are gases at the room temperature; the alkanes having 5 to 17 carbon atoms per molecule are liquids whereas those containing 18 or more carbon atoms per molecule, are solids at the room temperature.

According to this classification :

The molecular formulae of three hydrocarbons A, B and C are CH₄, C ₃H₈ and C₈H₁₈ respectively. Which of these hydrocarbons would occur as liquid at room temperature ?

We know that the hydrocarbons having 5 to 17 carbon atoms per molecule occur as liquids at the room temperature.

Here, the hydrocarbon C has 8 carbon atoms in its molecule (its molecular formula being C₈H₁₈), therefore, hydrocarbon C would occur as liquid at the room temperature.

The hydrocarbon A (CH₄) contains only one carbon atom in its molecule whereas hydrocarbon B (C₃H₈) contains 3 carbon atoms in its molecule.

So, both A and B will occur as gases at the room temperature.