Alkenes with a mass of 0.7 g decolorize 1.6 g of bromine water. Determine the molecular weight of the alkene.

m (alkene) = 0.7 g.

m (Br2) = 1.6 g.

Let’s determine the molar mass of alkenes.

The formula for alkenes is CnH2n

We write down the equation of reactions.

CnH2n + Br2 = CnH2n-1Br + HBr

Find the molar mass of the alkene.

M (СnH2n) = 12 * n + 1 * 2n = 12n + 2n = 14n

M (Br2) = 80 * 2 = 160 g / mol

Further, using the reaction equation, we find the unknown value of n.

0.7 g alkene – 1.6 g bromine

14n alkene – 160 g / mol bromine

0.7 * 160 = 1.6 * 14n

112 = 22.4n

n = 112 / 22.4 = 5.

We write down the alkene formula.

CnH2n = C5H10

M (C5H10) = 12 * 5 + 1 * 10 = 60 + 10 = 70 g / mol

Answer: the molar mass of alkene is 70 g / mol.