Anonymous
Anonymous asked in Science & MathematicsChemistry · 2 years ago

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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?

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  • 2 years ago
    Favorite Answer

    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

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