This document provides information about an alkene called Y, including its elemental composition by percentage and relative molecular mass. It asks to determine the molecular formula, draw the structural formula, name the alkene, and write its homologous series general formula. It also describes a test differentiating alkene Y from propane by adding bromine water to each, where bromine water is decolorized for alkene Y but not propane. This is because alkene Y contains an unsaturated carbon-carbon double bond that undergoes addition, while propane is saturated so no reaction occurs.

1. SPM Corner
The information below is regarding alkene Y:
 Carbon 85.7%
 Hydrogen 14.3%
 Relative molecular mass = 42
 Relative atomic mass of H = 1 and C = 12
Based on the information of the alkene Y:
(i) Determine the molecular formula
(ii) Draw the structural formula
(iii) Name the alkene
(iv) Write the general formula for its homologous series.
(i) Table 2.2 shows the results of a test to differentiate between alkene Y and propane. [8 marks]
Procedure Observation
Bromine water is added to alkene Y Brown colour is decolourized
Bromine water is added to propane Brown colour remains
Table 2.2
Explain why there is difference in these observations. [4 marks]
ANSWER
(i)
Element Carbon Hydrogen
Number of moles 85.7 14.3
12 = 7.083 1 = 14.1
Simplest ratio 7.083 14.1
:
7.083 7.083
1:2
Empirical formula = CH2
Molecular formula of alkene Y : (CH2)n
Given relative molecular mass = 42
Hence 12n + 2n = 42
14n = 42
n=3
 molecular formula of alkene Y is C3H6
(ii)
(iii) Propene
(iv) CnH2n
(i) 1. Alkene Y is an unsaturated hydrocarbon with a carbon-carbon double bond.
2. When bromine water is added to alkene Y, brown colour is decolourised because
addition reaction (or bromination ) occurs.
3. Propane is a saturated hydrocarbon with carbon – carbon single bond.
4. No reaction occurs when bromine is added.