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Homework Help: The combustion of petrol can be approximately represented by the

  1. Jun 13, 2010 #1
    The combustion of petrol can be approximately represented by the equation

    2C8H18(g) + 25O2 -----> 16CO2 + 18H2O

    The density of Ocatane is 0.7 gcm(cubed)

    What mass of oxygen is used up when 1L of petrol is burnt in a car engine?


    0.7 gcm[tex]^{}-3[/tex] = 0.0007 gL[tex]^{}-1[/tex]
    density = mass/volume
    0.0007 gL[tex]^{}-1[/tex] = mass / 1 L
    0.0007g = mass

    amount (of petrol) = mass/molar mass
    = 0.0007/228
    = 0.000003 mols

    therefore...amount of oxygen is 25 times amount i got for petrol?
    n(o) = 0.000003*25
    0.000075 = mass/molar mass
    0.000075 = mass/32
    0.0024g = mass

    Have i blown it somewhere? Im not sure about the mols thing i did when going from petrol to oxygen... I mean do you times it by 25 OR 12.5 because the mol ratio is 2petrol + 25Oxygen
    i think times by 25 because when finding molar mass for petrol I multiplied everything by 2

    btw the right answer is appparentlyyyy 2.5*10^3 grams, what did i do? ahh
  2. jcsd
  3. Jun 14, 2010 #2


    User Avatar

    Staff: Mentor

    Re: stoichiometry

    Mass of 1L is not 0.0007g.

    Please use [noparse]cm3[/noparse] tags for cm3 and [noparse]H2[/noparse] for H2.
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