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Homework Help: Sig figs chemistry

  1. Oct 16, 2014 #1
    1. The problem statement, all variables and given/known data
    Calculate what mass of oxygen is required to completely combust 500.0g of gasoline. Assume that the gasoline contains only octane.

    2. Relevant equations

    My problem is... I don't know WHEN to apply sig figs. If this is a multi-step problem (i to iv in this case), do I apply sig figs starting in i)? if I do, then all of my future answers will be influenced by using sig figs early on. When I get to iv) I don't know if all my digits are OK or if they're all way off. Can someone please clean the sig figs up a bit and tell me when I need to apply them? thank you!

    3. The attempt at a solution
    i) 2C(8)H(18) +25O(2) = 16CO(2) +18H(2)O

    ii) 500.0g/114.22852 = 4.377 mol of C(8)H18)

    iii) z=4.377 *25/2
    z=54.71488206 mol
    z=55 mol of O(2)

    iv) m(O(2))=55mol*31.9988g/mol
    m(O(2))=1759.934 g

    m(O(2))=1760 g
  2. jcsd
  3. Oct 16, 2014 #2


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    Staff: Mentor

    Do all calculations using full precision. Just the numbers you report should be rounded down.

    As the initial value has 4 sigfigs, all your results should be reported to 4 sigfigs as well (hence 54.71 moles of oxygen, and not 55 moles).
  4. Oct 16, 2014 #3
    ok, so you're saying that the initial 500.0 stems down through the entire series of steps to the question and 4 sig figs will always be required?
    Last edited: Oct 16, 2014
  5. Oct 16, 2014 #4


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    Staff: Mentor

    As it is the only number here - yes.

    (That is - there are other numbers, like molar masses, but we typically know them with much higher accuracy).
  6. Oct 17, 2014 #5
    ok thank you very helpful!
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