[KSuran]

Hi, I have 2 separate questions that I am not too sure how to answer.

 The first one is related to entropy:

 1) Nitrogen(III) oxide, N2O3, an air pollutant, dissolves in rainwater to form a dilute solution of nitrous acid. The equation for the reaction is: N2O3(g) + H2O(l) → 2HNO2(aq)

 Calculate ΔS° for this reaction in J mol-1 K-1. Use data in the table below:

 Compound  | S °, (J mol-1 K-1)
 N2O3(g)    | 307
 H2O(l)       | 69.96
 HNO2(aq)  | 135.6

 I thought that:
 ΔS° = sum(S ° products) - sum(S ° reactants)
       = 135.6 - (307 + 69.96)
       = -241.36 J mol-1 K-1
 But, this was a wrong answer. Where did I go wrong?

 The second question is related to solubility.

 2)
 a) When 25.0 mL of a solution of 1.4 x 10-3 M silver nitrate is mixed with 60.0 mL of a solution of 7.5 x 10-4 M sodium chloride, what is the molar concentration of silver ions in the final mixture?

 b) What is the molar concentration of chloride ions in the final mixture?

 Based on a comparison between Qsp and Ksp for this mixing reaction, a precipitate of silver chloride
 will/will not(?) form. (Ksp (AgCl) = 1.8 x 10-10)

 With this one, I calculated n(AgNO3) to be 3.5x10^-5 mol, and n(NaCl) to be 4.5x10^-5mol, but I do not know what to do next? I thought that the number of moles of each should be the same because of the molar ratios in the equation, so I'm lost.

 Thank you for your help.

[UG]

I thought that:
 ΔS° = sum(S ° products) - sum(S ° reactants)
       = 135.6 - (307 + 69.96)
       = -241.36 J mol-1 K-1
 But, this was a wrong answer. Where did I go wrong?

You forgot about the stoichiometric coefficients. You need to multiply 135.6 by 2.

For 2 a) the question asks for the molar concentrations in the mixture, can you calculate these first?

[Borek]

For the second problem - AgCl will precipitate removing Ag⁺ and Cl^- from the solution till the product of remaining concentrations will equal Ksp. You know from the stoichiometry that [Ag⁺]_removed=[Cl^-]_removed.

[KSuran]

I thought that:
 ΔS° = sum(S ° products) - sum(S ° reactants)
       = 135.6 - (307 + 69.96)
       = -241.36 J mol-1 K-1
 But, this was a wrong answer. Where did I go wrong?

You forgot about the stoichiometric coefficients. You need to multiply 135.6 by 2.

For 2 a) the question asks for the molar concentrations in the mixture, can you calculate these first?

So, ΔS° = sum(S ° products) - sum(S ° reactants)
       = 2(135.6) - (307 + 69.96)
       = -105.76 J mol-1 K-1   ??

I submitted this answer and the program still says I am incorrect. Do you have any idea of what is wrong with this value?