[Rutherford]

A 12.25 g sample of an equimolar mixture of two unknown salts (both of which are stable at room temperature and pressure) was sealed in a 10.00 L vessel. The vessel was evacuated and heated to 100 °C. The pressure inside the vessel measured at this temperature was found to be 93.00 kPa. After the vessel was cooled to 20 °C, the pressure dropped to 24.35 kPa. The vessel was opened and the condensate (m=9.45g) was completely dissolved in 200 mL of distilled water. A litmus test of the resulting clear solution showed a neutral reaction. Addition of an excess of aqueous lead(II) nitrate to this solution produced 13.9 g of a precipitate which is 74.48% lead by mass.

There is an answer, but I don't get the logic, so I want to start this from zero.
Number of moles of the gasses at 100°C is 0.3mol.
Number of moles of the gasses at 20°C is 0.1mol, so there was 0.2mol of water, i.e. 3.6g and 0.1mol of a gas.
The precipitate is PbCl₂ and n_Cl^-=0.1mol and its mass is 3.55g.
Here I am stuck. Where to continue?

[Rutherford]

I got further. The mass of the unknown gas is 12.25g-9.45g=2.8g as there was 0.1mol of it, the gas has a molar mass of 28g/mol, N₂ or CO. I can't think of a reaction where two salts react to form CO, so I will guess it's N₂.

In the salt mixture there were: 4H, 2O, 2N, 1Cl, and X. As N₂ is produced, the salts should be NH₄Cl and XNO₂ (0.1mol of each) or vice versa (metal chloride and ammonium nitrite). The metal has to be monovalent (as 0.1mol of NO₂^- and 0.1mol of NH₄⁺ will give 0.1mol of N₂).

So, the metal left in the condensate, too (it's n=0.1mol). It's mass is m=9.45-(3.6+3.55)=2.3g, M=23g/mol, it is Na.
NaNO₂ and NH₄Cl or NaCl and NH₄NO₂, how to decide which pair was it? Where is the clue that helps me determining between these two?

[Rutherford]

Anyone?

I got that the unknown salts are either NaNO₂ and NH₄Cl or NaCl and NH₄NO₂. How to deduce which pair was in the initial mixture? Where is the tip in the problem that helps deducing if there is any?
By the way, the answer is NaNO₂ and NH₄Cl, but why?

I would be really grateful for help.

[Rutherford]

Okay, so no info in the problem was given that could help me determine this.
Then I have another question: What info could help me to distinguish between these two pairs (that the answer is NaNO₂ and NH₄Cl)? I am asking this because I like this problem and I want it to be correct, so I can print it. I hope that someone answers, but if not I won't revive this again.

[AWK]

Hint:
BOTH ARE STABLE AT ROOM TEMPERATURE.

[Rutherford]

I suppose that NH₄NO₂ is the unstable one and that it start decomposing even on room temperature, but why would an ionic compound behave so?

[discodermolide]

What happens when you acidify them? Or indeed make them basic?

[Rutherford]

I don't understand. Could you explain what you meant?

[discodermolide]

Well take a solution of sodium nitrite and add acid, HCl, what will happen?
Take a solution of ammonium chloride and add NaOH solution, what will happen?

[Rutherford]

Neutralization in both situations, but if it were NaCl and NH₄NO₂ only NH₄NO₂ would react. How could I formulate this?

[discodermolide]

Surely in the one case you will get NO₂ and in the other ammonia?

[Arkcon]

Hint:
BOTH ARE STABLE AT ROOM TEMPERATURE.

But when heated to 100 C, they apparently react.  Look at the chemicals involved: NaNO2, NH4Cl,  NaCl and NH4NO2 -- one of these is not like the others.

[Rutherford]

Surely in the one case you will get NO₂ and in the other ammonia?

NH₄Cl with NaOH gives ammonia, NaNO₂ with HCl gives HNO₂, NH₄NO₂ with NaOH would give ammonia and NaCl won't react with bases or acids. Did you mean this?

Hint:
BOTH ARE STABLE AT ROOM TEMPERATURE.

But when heated to 100 C, they apparently react.  Look at the chemicals involved: NaNO2, NH4Cl,  NaCl and NH4NO2 -- one of these is not like the others.

In the first case: NaNO₂+NH₄Cl N₂+NaCl+2H₂O;
In the second case: NH₄NO₂ N₂+2H₂O while NaCl remains unreacted. In both cases the products are same, so I don't see a way to distinguish between the two possible pairs.

[discodermolide]

Throw in NaN₃ to the mixture and then acidify, what do you get?

[Rutherford]

Don't know. Whit which of the 4 possible salts would it react?