[mburt]

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 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

[Dan]

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.

[mburt]

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?

[Dan]

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.

[juanrga]

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 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.

[mburt]

Okay thanks! But I agree with the earlier poster that the significant figures would only be based on the accuracy of the molar masses.

[camptzak]

I feel like every good chemist knows, rules are made to be broken!

except lab saftey rules

[juanrga]

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

[mburt]

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.