1 g of an alkene that contains only one double bond per molecule reacted completely with 8.0 g of bromine, Br2 - VCE - SSCE Chemistry - Question 18 - 2012 - Paper 1

Since 1 mole of alkene reacts with 1 mole of bromine, the number of moles of alkene is equal to the number of moles of bromine reacted:

$n_{alkene} = n_{Br2} = 0.05 \, mol$.

The molar mass of the alkene can be calculated using the mass of the alkene and its moles:

$M = \frac{mass}{n}$

Given the mass of alkene is 1 g:

$M_{alkene} = \frac{1 \, g}{0.05 \, mol} = 20 \, g \ mol^{-1}$.

The general formula for alkenes is $C_nH_{2n}$. Setting up the equation for the molar mass:

$12n + 2n = 20 \, g \ mol^{-1}$ spans to:

$14n = 20 \Rightarrow n = \frac{20}{14} \approx 1.43$,

which we round down to the nearest whole number, giving us n = 2. Thus:

The molecular formula of the alkene is $C_2H_4$.