Sig Figs in Chemistry: Calculate Mass of O2 from Gasoline

[ilii]

Homework Statement

Calculate what mass of oxygen is required to completely combust 500.0g of gasoline. Assume that the gasoline contains only octane.[/B]

Homework 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!

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[/B]
m(O(2))=1760 g

 

[Borek]

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).

 

[ilii]

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?

 

[Borek]

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).

 

[ilii]

ok thank you very helpful!