[Atome]

Hello there,

Could anyone please see where I may have erred for this following problem?

Thank you.

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1. Consider the equilibrium: 2NO_{2 (g)}  N₂O_{4 (g)}.

Some of the equilibrium mixture was drawn into a 500 mL syringe whose mass, when containing air @ SATP is 124.45 g. When filled with the above equilibrium mixture the mass of the syringe was 125.12 g @ 30⁰C at 755 mmHg.

Calculate K_c for the above equilibrium.

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My Work:

Since the original syringe includes air, the mass of air must be subtracted from 124.45 g to determine the mass of the empty syringe.

Density of air = 1.2 g/L.

Therefore: Mass of air = 1.2 g/L x 0.5 L = 0.60 g.

Mass of empty syringe = 124.45 - 0.60 g = 123.85 g.

Mass of NO₂ + N₂O₄ = 125.12 g - 123.85 g = 1.27 g

Mol of NO₂ + N₂O₄ = PV/RT = (100.66 kPa x 0.500 L)/(8.314 x 303 K)

= 0.01998 mol

Let x = mol NO₂ and (0.01998 - x) = mol N₂O₄.

46x + 92(0.01998 - x) = 1.27 g

x = 0.0124 mol and 0.01998 - x = 0.00758.

K_c = [N₂O₄]/[NO₂]²

                   = (0.00758 mol/0.5 L)/(0.0124 mol/0.5)²

                   = 25 @ 303 K, 100.66 kPa

However, the correct answer should be around 12.

[Borek]

Density of air = 1.2 g/L.

Close, but that's only an approximated value, you will need more precise one. Not sure if it will be enough for twofold change of the final result. I have only skimmed your post and it seems like your approach is correct, but I could miss something.

[Atome]

Thanks for your reply, Borek.