September 30, 2022, 02:30:46 AM
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Topic: Computing the mass of CaF2 and Enthalpy of phase change of OsO4(s) to OsO4(g)  (Read 934 times)

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Offline Win,odd Dhamnekar

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1)Calculate the mass of CaF2 produced when 5g of Ca(NO3)2 is mixed with 3g of NaF in water, with a final volume of 500 mL. Ksp is 3.5E-11 for the system.
 My attempt to answer: Reaction equation is Ca(NO3)2 + 2NaF  :rarrow: CaF2 + 2NaNO3
We have 5/164.088 moles of Ca(NO3)2 and 3/41.9882 moles of NaF

How to answer this question?

2)What is the intention of the questioner to include highlighted portion in the question?
(Note:You can see that the highlighted portion of information is not useful in answering this question.)
« Last Edit: July 05, 2021, 08:31:56 AM by Win,odd Dhamnekar »
Any science consists of the following process.
 1) See 2) Hear 3) Smell if needed 4) Taste if needed
5) Think 6) Understand 7) Inference 8) take decision [Believe or disbelieve, useful or useless, healthy or unhealthy, cause or effect, favorable or unfavorable, practical or theoretical, practically possible or practically impossible, true or false or  any other required criteria]

Offline Orcio_87

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Quote
1)Calculate the mass of CaF2 produced when 5g of Ca(NO3)2 is mixed with 3g of NaF in water, with a final volume of 500 mL. Ksp is 3.5E-11 for the system.
 My attempt to answer: Reaction equation is Ca(NO3)2 + 2NaF  :rarrow: CaF2 + 2NaNO3
We have 5/164.088 moles of Ca(NOF3)2 and 3/41.9882 moles of NaF
How to answer this question?
1. Calculate mass of precipitated CaF2.
2. Using solubility product calculate how much Ca2+ can dissolve in NaF solution.
3. Mass of CaF2 residue is reduced by this soluble part.

Quote
2)What is the intention of the questioner to include highlighted portion in the question?
(Note:You can see that the highlighted portion of information is not useful in answering this question.)
It is direct answer that formation of solid OsO4 is more exotermic - nothing more.

Offline Win,odd Dhamnekar

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CaF2 ::equil:: Ca2+ +2F-, Ksp=[Ca2+] + [2F-]2 [itex]\implies[/itex] ,Q=[0.061]*[2*0.143]2=4.97e-3

Q> Ksp, , so, CaF2 will precipitate if its mass > 2.38 grams

Are  this workings correct answers to your point 2) and 3)?
Any science consists of the following process.
 1) See 2) Hear 3) Smell if needed 4) Taste if needed
5) Think 6) Understand 7) Inference 8) take decision [Believe or disbelieve, useful or useless, healthy or unhealthy, cause or effect, favorable or unfavorable, practical or theoretical, practically possible or practically impossible, true or false or  any other required criteria]

Offline Orcio_87

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Solubility of CaF2 is very low (does not change mass of the residue), so you are right.

Offline mjc123

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Ksp = [Ca2+][F-]2. Do not multiply the concentration of F- by 2!
If, and only if, CaF2 is the only source of calcium and fluoride ions, then if the concentration of CaF2 is C, [Ca2+] = C and [F-] = 2C (that's where a factor of 2 comes in) and Ksp = 4C3. You can't use this when there are other sources of the ions.

Where do you get the figure of 2.38 g? I calculate the solubility to be 8 mg in 500 mL. (confirmed by looking it up)

Offline Orcio_87

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@mjc123 - 2,38 g is the mass of precipitate.

Offline mjc123

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Oh, I see. Then the statement "CaF2 will precipitate if its mass > 2.38 grams" is wrong.

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