Determining an Emperical Formula using Combustion Analysis data

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Discussion Overview

The discussion revolves around determining empirical formulas using combustion analysis data for two compounds: naphthalene and ethyl alcohol. Participants explore the calculation methods and ratios involved in deriving empirical formulas from combustion products.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant presents a problem involving naphthalene and shares their calculations for moles of carbon and hydrogen, questioning how to determine the appropriate fraction for ratios.
  • Another participant clarifies that 0.8 can be expressed as 4/5, suggesting that some fractions are straightforward.
  • A participant expresses uncertainty about when to multiply ratios by fractions, asking for a rule of thumb for less obvious cases.
  • Another participant suggests a practical approach of multiplying by integers until whole numbers are reached.
  • A different problem involving ethyl alcohol is introduced, where a participant calculates the empirical formula but questions the divisor used for determining the ratio of elements, noting a discrepancy with the book's answer.
  • A participant prompts for a calculation check on the moles of carbon and oxygen to clarify the empirical formula derivation.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the method for determining appropriate fractions in empirical formula calculations. There is disagreement on the correct approach to deriving the empirical formula for ethyl alcohol, with differing results between the participant's calculations and the book's answer.

Contextual Notes

Participants have not reached a consensus on the rules for determining when to multiply ratios by fractions, and there are unresolved calculations regarding the empirical formula for ethyl alcohol.

Who May Find This Useful

This discussion may be useful for students and individuals interested in chemistry, particularly those studying combustion analysis and empirical formula calculations.

Crookedsky
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the problem I am working with is:
Naphthalene is a carbon-hydrogen compound that finds use as mothballs. A sample of naphthalene is subjected to combustion analysis, producing 1.100g of CO2 and .1802g of H2O. Based on these data, calculate the empirical formula of naphthalene.

So when i set up the demensional analysis and get the moles of C and H i get for C .02499/
.02499 and for H i get .02000/.02499. For H the book multiplies the fraction for H by 5 because .02000/.02499 = .800320128 which is not close to a whole number.

My question is: How do they determine what the fraction is. Which is in this case is 4/5ths.

The Emperical Formula for napthalene is C5H4
 
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0.8=8/10=4/5.

It is not always that obvious, but here it is pretty simple.
 
Ya but how do i know when to multiply is by some fraction? Like .99999 is pretty obvious but what about .4154 or .5900 etc etc. What is the rule of thumb?
 
Crookedsky said:
Ya but how do i know when to multiply is by some fraction? Like .99999 is pretty obvious but what about .4154 or .5900 etc etc. What is the rule of thumb?

At a simple level, just multiply by 2, 3, 4, 5 etc until you get reasonably close to whole numbers
 
Ok thanks Sjb,
Now i have a different problem that i have been working:
Ethyl Alcohol, the alcohol present in alcoholic beverages, is a carbon-hydrogen-oxygen compound. Combustion analysis of a 1.000g sample of ethyl alcohol produces 1.913g of CO2 and 1.174g of H2O. Based on these data, calculate the empirical formula of ethyl alcohol.

So i set the dimensional analysis and solve for grams of Carbon(.5220) and grams of Hydrogen(.1316). Then i add C and H up and get .6536g mass. Next i solve for O by sutracting .6536 from 1.000 and come up with a .3464g of O. Then I convert H, C, and O to moles and come up with .04346 mol of C, .130 mol of H, and .02165 mol of O.

At this point either the book is wrong or i am wrong. I used the moles of C (.04346) as my divisor to calculate the ratio. I came up with an empirical formula of CH3O5 but the book uses the moles of O (.02165) as the divisor and comes up with a empirical formula of C2H6O.

Can someone check my calculations because i have done this problem twice. Thanks
 
What is .04346 / .02165 ?
 
Numbers of moles you calculated so far are OK. Answer sjb's question.
 

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