Calculate the Percent Composition of an Herbicide from Analysis Data?

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Discussion Overview

The discussion revolves around calculating the percent composition of an herbicide based on combustion analysis data. Participants explore the methodology for determining the mass of various elements (C, H, N, Cl) in the herbicide sample, utilizing stoichiometric relationships and gas laws.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a combustion analysis problem involving an herbicide containing C, H, N, and Cl, providing specific data for calculations.
  • Another participant suggests that percent composition can be calculated using mass/mass ratios rather than moles for chlorine.
  • Questions arise regarding how to determine the mass of the other elements in the herbicide sample.
  • Participants discuss the calculation of moles for CO2 and H2O using the ideal gas law, with one participant providing detailed calculations for moles of C and H.
  • There is a claim that the method used for calculating the mass of carbon and hydrogen appears correct, although arithmetic verification is not performed.
  • A later reply challenges the interpretation of the relationship between moles of CO2 and moles of carbon, asserting that one mole of CO2 contains one mole of carbon.

Areas of Agreement / Disagreement

Participants express differing views on the approach to calculating percent composition, with some agreeing on the method while others contest specific interpretations of the stoichiometric relationships involved. The discussion remains unresolved regarding the correctness of the calculations and the methodology used.

Contextual Notes

Limitations include potential arithmetic errors in the calculations presented, as well as assumptions regarding the completeness of combustion and the ideal behavior of gases under the given conditions.

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Homework Statement


An herbicide is found to contain only C, H, N, and Cl. The complete combustion of a 100.0-mg sample of the herbicide in excess oxygen produces 83.16 mL of CO2 and 73.30 mL of H2O vapor at STP. A separate analysis shows that the sample also contains 16.44 mg of Cl.

Determine the percent composition of the substance.


Homework Equations



pV = nRT

The Attempt at a Solution



moles of Cl = .01644/35.453
 
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You don't need moles of chlorine. Percent composition means mass/mass.
 
So how do I get the mass of the other elements?
 
From moles. In the case of chlorine mass was given straight away.
 
How do I figure out the number of moles for the other elements?
 
Using equation that you already posted.

Thats enough spoonfeeding, you either start to think on your own, or I am not going to help any further.
 
moles of CO2 = pV/RT = 0.08316/(0.0821 * 273) = 0.003710297 moles
Moles of C = (12/44) * 0.003710297 = 0.00101199 moles
mg of C = (0.00101199)*(12000) = 12.14mg

moles of H2O = pV/RT = 0.07330/(0.0821 * 273) = 0.00327038 moles
moles of H = (2/18) * 0.00327038 = 0.000363376 moles
mg of H = (0.000363376)*(1000) = 0.363375516 mg

mg of N = 100mg - 0.363375516 mg - 12.14mg - 16.44mg = 71.05 mg

So the % composition is
C 12.14%
H 0.3634%
N 71.05%
Cl 16.44%

Is that right?
 
The method looks good, ( I did not check for arithmetic computation mistakes ).
 
6021023 said:
moles of CO2 = pV/RT = 0.08316/(0.0821 * 273) = 0.003710297 moles
Moles of C = (12/44) * 0.003710297 = 0.00101199 moles

No. 1 mole of CO2 contains 1 mole of C.
 

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