How Much Energy Is Needed to Decompose 765g of PCl3?

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Discussion Overview

The discussion revolves around calculating the energy required to decompose 765 g of PCl3 based on a given chemical reaction and its enthalpy change. Participants are attempting to solve a homework problem involving stoichiometry and thermodynamics.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant calculates the number of moles of PCl3 from the given mass and then attempts to find the energy using the enthalpy change, arriving at a value of 6.72 x 103 kJ.
  • Another participant points out that the highlighted operators in the calculations are incorrect and emphasizes the importance of the number of moles in the enthalpy equation.
  • A participant questions the interpretation of the number of moles, asserting that they derived the value of 4 from the balanced equation and seeks clarification on their error.
  • Another participant reiterates the molar mass of PCl3 as 137.32 g/mol and prompts for the calculation of moles in 765 g.

Areas of Agreement / Disagreement

There is no consensus on the correct approach to the problem, as participants express differing views on the calculations and the interpretation of the enthalpy change. The discussion remains unresolved.

Contextual Notes

Participants have not reached an agreement on the correct method for calculating the energy, and there are indications of potential misunderstandings regarding the stoichiometric coefficients and their application in the enthalpy calculation.

KTiaam
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Homework Statement


How much energy is required to decompose 765 g of PCl3, according to the reaction below? The molar mass of PCl3 is 137.32 g/mol and may be useful.

4 PCl3(g) → P4(s) + 6 Cl2(g) ΔH°rxn = +1207 kJ

The Attempt at a Solution



765 g PCl3 x 4 mol PCl3/137.32 g PCl3 = 22.28 mol PCl3

22.28 mol PCL3 x 1207 kJ/4 mol PCl3 = 6.72 x 103 kJ

Which is incorrect...I converted from grams to mols then i converted mols to kJ, which did not give me the correct answer.
Im probably using a wrong sign somewhere or completely approaching the problem correctly.

Thank you for your help
 
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KTiaam said:

Homework Statement


How much energy is required to decompose 765 g of PCl3, according to the reaction below? The molar mass of PCl3 is 137.32 g/mol and may be useful.

4 PCl3(g) → P4(s) + 6 Cl2(g) ΔH°rxn = +1207 kJ

The Attempt at a Solution



765 g PCl3 x 4 mol PCl3/137.32 g PCl3 = 22.28 mol PCl3

22.28 mol PCL3 x 1207 kJ/4 mol PCl3 = 6.72 x 103 kJ

Which is incorrect...I converted from grams to mols then i converted mols to kJ, which did not give me the correct answer.
Im probably using a wrong sign somewhere or completely approaching the problem correctly.

Thank you for your help

The highlighted operators are incorrect. Pay attention to number of moles in the equation for enthalpy.
 
Bystander said:
The highlighted operators are incorrect. Pay attention to number of moles in the equation for enthalpy.

the number of moles is 4? I got 4 from the balanced equation?could you explain more on what I'm doing wrong?
 
Step at a time: PCL3 is 137.32 gram per mole (gram mole, gram molecular weight), also written 137.32 g/mol. How many moles in 765 g?
 

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