[Fish200398]

Given : A sufficient amount of chlorine (Cl2) underwent an addition reaction with a certain
amount of alkene A. The mass of the product was 140 g heavier than that of A. Then a
sufficient amount of bromine (Br2) underwent addition to the same amount of A. The mass of
the product was 460 g. Whats the correct one as the number of
carbon atoms of A.
Given also :
Mr Cl2 = 35 g
Mr Br = 80
Key answer : 5

The product was 140 g heavier than that of A. But mass of Cl2 is 70 g, what does it mean?

[chenbeier]

That means 2 mol Cl₂ was added.

[Fish200398]

Oh yes, thats makes sense

[chenbeier]

So now calculate further. What does it mean how many mol bromine. What mass was added and what is the mass of A.
From this you can calculate how much mole A and also the amount of C- atoms.

[Fish200398]

Yes, thanks

[chenbeier]

No you should post your calculation.

[Fish200398]

Known : 2 moles Cl2 reacted with Alkene named A, it means the Alkene reacted also 2 moles,

Because in halogenation the mole ratio of

alkene : halogen : product = 1:1:1

2 moles of A reacted with Br2, has product of 460 g

Mass of A + mass of Br2 = 460 g
Mass of A = 460 - 320 = 140 g

2 moles of A has mass of 140 g.
Mr A = 70 g/moles
Alkene A is C5H10

[chenbeier]

Yes correct calculation.