Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Win,odd Dhamnekar on July 05, 2021, 06:21:24 AM

Title: Computing the mass of CaF2 and Enthalpy of phase change of OsO4(s) to OsO4(g)
Post by: Win,odd Dhamnekar on July 05, 2021, 06:21:24 AM

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. K_sp 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.)

Title: Re: Computing the mass of CaF2 and Enthalpy pf phase change of OsO4(s) to OsO4(g)
Post by: Orcio_87 on July 05, 2021, 08:05:57 AM

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.

Title: Re: Computing the mass of CaF2 and Enthalpy of phase change of OsO4(s) to OsO4(g)
Post by: Win,odd Dhamnekar on July 06, 2021, 02:25:03 AM

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

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

Are  this workings correct answers to your point 2) and 3)?

Title: Re: Computing the mass of CaF2 and Enthalpy of phase change of OsO4(s) to OsO4(g)
Post by: Orcio_87 on July 06, 2021, 07:41:32 AM

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

Title: Re: Computing the mass of CaF2 and Enthalpy of phase change of OsO4(s) to OsO4(g)
Post by: mjc123 on July 06, 2021, 08:46:36 AM

K_sp = [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 K_sp = 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)

Title: Re: Computing the mass of CaF2 and Enthalpy of phase change of OsO4(s) to OsO4(g)
Post by: Orcio_87 on July 06, 2021, 09:05:21 AM

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

Title: Re: Computing the mass of CaF2 and Enthalpy of phase change of OsO4(s) to OsO4(g)
Post by: mjc123 on July 06, 2021, 09:12:11 AM

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