C6H10 + Br2 = C6H10Br2
cM = n/V=> n = cM * V = 1 * (20/1000) = 0.02 moli Br2
1 mol ciclohexena...........1 mol brom
x moli ciclohexena.............0.02 moli brom
x = 0.02 moli ciclohexena
masa ciclohexena = 0.02 * 82 = 1,64 g
puritatea = masa pura/masa impura * 100 = 1.64/1.8 * 100 = 91.11%