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Topic: Review this Stoichiometry Question Please!  (Read 5679 times)

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Offline mburt

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Review this Stoichiometry Question Please!
« on: July 11, 2012, 06:25:31 PM »
Hey guys,

I did a question in my chem textbook tonight and no answer is supplied so I thought I might try posting it here. If I'm wrong or if you have any advice please let me know!

Question:

In the combustion of octane, C8H18, how many grams of O2 are consumed per gram of octane?

My answer:

2C8H18 + 25O2 :rarrow: 16CO2 + 18H2O

By the way: I just chose theta, θ, as a variable. "x" didn't look very clear

θg C8H18 × 1mol C8H18 / 114.2278g C8H18 × 25mol O2 / 2mol C8H18 × 31.9988g O2 / 1mol O2 = 3.5θ g O2

Therefore 3.5g of O2 are consumed per gram of octane.

I wasn't sure about the significant digits for this one since the question deals with arbitrary numbers.

Thanks,

Mike

Offline Dan

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Re: Review this Stoichiometry Question Please!
« Reply #1 on: July 11, 2012, 08:11:14 PM »
I agree with 3.5. Also not sure about the sig figs, I guess it depends on the accuracy of the molar masses supplied in the question. 2 sig figs is sensible, the important thing is that the method is correct.
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Offline mburt

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Re: Review this Stoichiometry Question Please!
« Reply #2 on: July 11, 2012, 09:06:16 PM »
Thanks for your response. In some ways I'm glad that the textbook doesn't supply answers all the time- it keeps me disciplined and honest about my answers.

By the way, I noticed that you are a moderator; is it against the rules to ask for answers to questions?

Offline Dan

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Re: Review this Stoichiometry Question Please!
« Reply #3 on: July 12, 2012, 03:07:41 AM »
Quote
is it against the rules to ask for answers to questions?

It is against the rules to ask for help without showing that you have attempted the question yourself. People replying to your question are not supposed to give you the final answer, but should guide you through the method so you find it yourself.
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Offline juanrga

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Re: Review this Stoichiometry Question Please!
« Reply #4 on: July 15, 2012, 09:31:27 AM »
Hey guys,

I did a question in my chem textbook tonight and no answer is supplied so I thought I might try posting it here. If I'm wrong or if you have any advice please let me know!

Question:

In the combustion of octane, C8H18, how many grams of O2 are consumed per gram of octane?

My answer:

2C8H18 + 25O2 :rarrow: 16CO2 + 18H2O

By the way: I just chose theta, θ, as a variable. "x" didn't look very clear

θg C8H18 × 1mol C8H18 / 114.2278g C8H18 × 25mol O2 / 2mol C8H18 × 31.9988g O2 / 1mol O2 = 3.5θ g O2

Therefore 3.5g of O2 are consumed per gram of octane.

I wasn't sure about the significant digits for this one since the question deals with arbitrary numbers.

Thanks,

Mike

In principle you would give 5 decimal figures instead of only one figure, because you are using data with 5 decimal figures.
Sharing unified knowledge in pure and applied sciences

Offline mburt

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Re: Review this Stoichiometry Question Please!
« Reply #5 on: July 15, 2012, 06:24:50 PM »
Okay thanks! But I agree with the earlier poster that the significant figures would only be based on the accuracy of the molar masses.

Offline camptzak

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Re: Review this Stoichiometry Question Please!
« Reply #6 on: July 16, 2012, 01:48:11 AM »
I feel like every good chemist knows, rules are made to be broken! ;D

except lab saftey rules 8)
"Chance favors the prepared mind"
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Offline juanrga

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Re: Review this Stoichiometry Question Please!
« Reply #7 on: July 16, 2012, 09:35:53 AM »
Okay thanks! But I agree with the earlier poster that the significant figures would only be based on the accuracy of the molar masses.

That is what I mean by "you would give 5 decimal figures instead of only one figure". The 31.9988 g·mol-1 O2 has 5 decimal figures: 31.9988 = 3.19988 · 10-1
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Offline mburt

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Re: Review this Stoichiometry Question Please!
« Reply #8 on: July 16, 2012, 07:09:20 PM »
Oh alright. Thanks. In terms of realistically presenting numbers, I generally keep my answers between 0.1 and 1000. I understand that proper significant digits would require exponents.

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