Chemistry Help (on behalf of someone else)

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Discussion Overview

The discussion revolves around solving chemistry problems related to the precipitation of calcium fluoride (CaF2) and the calculation of enthalpy of formation for phenol (C6H5OH) using Hess's law. Participants seek assistance with specific calculations and the application of thermodynamic principles.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant asks for the concentration of F- at which CaF2 will begin to precipitate in a 0.1 M Ca(NO3)2 solution, referencing the Ksp of CaF2 as 4e-11.
  • The same participant inquires about the solubility of CaF2 in the given solution.
  • Another participant suggests that the original poster should guide their friend on how to use forums effectively instead of seeking direct answers.
  • A later post presents a different chemistry problem involving the enthalpy of formation for C6H5OH, where the participant describes their calculations using Hess's law but indicates that their answer does not match the expected result.
  • The participant also presents a second problem regarding the temperature at which a non-spontaneous reaction becomes spontaneous, detailing their calculations and expressing confusion over the discrepancy in the expected answer.
  • Another participant challenges the calculations by pointing out that the third equation in the Hess's law application was ignored and requests clarification on the reaction equation derived from the calculations.

Areas of Agreement / Disagreement

The discussion contains multiple competing views and unresolved calculations. Participants express differing opinions on the approach to solving the problems, and there is no consensus on the correct answers or methods.

Contextual Notes

Participants have not provided all necessary assumptions or details for the calculations, and there are unresolved steps in the mathematical reasoning presented.

rizwaan
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1) CaF2 is added to a 0.1 M Ca(NO3)2 solution. At what concentration of F- will CaF2 begin to precipitate? Ksp of CaF2 is 4e-11??

2) The solubility of CaF2 (ksp = 4 e-11) in a 0.1 M solution of Ca(NO3)2 is approximately?


Need help finding the solutions for these. I'm posting this for someone else actually who's not that good with using forums. Please be as detailed as you can.

thank you very much in your advance for your help!
 
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You were told at chemical forums that nobody is going to do the question for you. You will do better service to your friend showing her/him how to use forums so that we can guide her/him through the question.
 
Hi I'm very sorry but I copied and pasted the wrong info.

This one has more of an explanation. Let me know if you need anymore info with this.

1) Given the reactions and thermodynamic data below, calculate the enthalphy of formation for C6H5OH kcal/mol.

(kcal)
C6H5OH + 7 O2 --> 6 CO 2 + 3 H2O 729.8
C + O2 ---> CO2 -94.4
2 H2 = O2 ---> 136.8

I used Hess's law and flipped the first equation and got -729.8 kcal and multiplied the second one by 6 to get 566.4 and added everything to get -26.6 kcal.
But this is not the right answer...apparently is +41.7?

2)
The balanced equation below is for a nonsponstaneous reaction (enthalpy of formation = 131 kj/mol and
entropy of formation = 134 J/(mol x k) ). Assuming that the change in enthalpy and entropy do not vary with temperature, at what temperature will the reaction become spontaneous?

C(s) + H2O(l) --->CO(g) + H2(g)

To solve this I used delat G = delta H - T(delta S).
I got 978 K and subtracted it by 273 to get 704 C. However the answer is 1022 C?

Thank you very much for your help.
 
rizwaan said:
I used Hess's law and flipped the first equation and got -729.8 kcal and multiplied the second one by 6 to get 566.4 and added everything

And you have ignored third equation? Show what you got (what reaction equation) after doing what you wrote you did.
 

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