Anonymous
Anonymous asked in Science & MathematicsChemistry · 2 years ago

Hey chemistry question requires a guide?

2.8g of a pure alkene X which contains only 1 double bond per molecule, reacts completely with 8g of bromine in CCl₄.Find the molecular formula for X?

Relevance
• 2 years ago

Equation:

R- CH = CH - R' + Br2 → R- CHBr - CHBr - R'

1mol alkene reacts with 1 mol Br2

Molar mass Br = 80g/mol : Br2 = 160g/mol

8g Br2 = 8/160 = 0.05 mol

Therefore 2.8g alkene = 0.05 mol

Molar mass alkene = 2.8/0.05 = 56g/mol

Alkene = CnH2n = 12 + 2 = 14g/unit

Molecular formula = 56/14 = 4 units/molecule n=4

Molecular formula = C4H8 butene

• Dr W
Lv 7
2 years ago

use dimensional analysis !!!'

'

the reaction looks like this

.. 1 R-CH=CH-R' + 1 Br2 ---> 1 R-CH(Br)-CH(Br)-R

then

.. 2.8g X.. .. 159.8g Br2.. .. 1 mol Br2

--- ----- --- x --- ---- --- ---- x --- ---- ---- = 55.9 g X / mol X

.. 8.0g Br .. .. 1 mol Br2.. .. 1 mol X

so we have this general formula CnH2n... (that's an alkene with 1 double bond) and let's look at possibilities

.. C1H2.. .. nope not enough bonds on the C

.. C2H4.. .. molar mass = 2x12.01 + 4x1.008 = 28.0 g/mol

.. C3H6.. .. molar mass = 3x12.01 + 6x1.008 = 42.1 g/mol

.. C4H8.. .. molar mass = 4x12.01 + 8x1.008 = 56.1 g/mol

.. C5H10.. .. molar mass = 5x12.01 + 10x1.008 = 70.1 g/mol

and we can see our unknown compound must be C4H8