[dummy_boy5]

Hey, guys!
Can someone explain how to solve this problem?

We have a reaction 3H₂+N₂  2NH₃

Initial concentrations  of N₂ = H₂ = 2 mol/L
Equilibrium concentration of NH₃ is 0.5 mol/L

So, I have to somehow calculate the equilibrium constant, but don't know how to do it...

[chenbeier]

1. Write the equation of the mass action law.
2. From the given equilibrium NH3 concentration read how much nitrogen and hydrogen was used.
3. Calculate the remaining concentration of nitrogen and hydrogen
4. Fill in the values in the mass action law

Now its your work to show

[dummy_boy5]

K = [NH₃]²/[N₂][H₂]³
I don't have the initial concentration of NH₃
You mean to calculate the remains concentrations I have to subtract from the initial concentrations like this:
[N₂] = 2 - 3×0.5 and [H₂] = 2 - 0.5 ?
Is it correct?

[chenbeier]

Mass action law is correct.
Of course you don't have initial concentration of ammonia because its 0.
Figure out how much hydrogen and nitrogen was used to get 0,5 mol NH3.
If you have that then subtract each from the initial values.
The results fill in mass action law.

[dummy_boy5]

I think it should be like that:
[N₂] = 2 - 1/2×0.5 = 1.75 and [H₂] = 2 - 3/2×0.5 = 1.25

and K would be: K = 0.5² / (1.75 × 1.25³) = 0.073

Its my intuition, but I think its not correct though...

[chenbeier]

Yes its correct you got it.

[dummy_boy5]

I really appreciate your help, thanks!

[chenbeier]

One thing, the unit is missing.

[Meter]

One thing, the unit is missing.

The equilibrium constant is unitless as it is implicitly understood that each concentration is divided by the standard concentration (1 mol/L), removing any units.

[chenbeier]

That is wrong. The unit depends on the equation.

In this case.

K = [NH3]2/[N2][H2]3

The unit is (l/mol)².

[Borek]

That is wrong. The unit depends on the equation.

In this case.

K = [NH3]2/[N2][H2]3

The unit is (l/mol)².

That's a lousy chemistry. How are you going to calculate [itex]\Delta G^0 = -RT \ln(K_p)[/itex] using K_p with unts?

Strict approach calls for unitless activities and for unitless equilibrium constant, what Meter wrote is perfectly correct.

[chenbeier]

That is what I learned.

https://www.toppr.com/ask/question/unit-of-equilibrium-constant-is/

[Borek]

Just because the error is common doesn't mean the idea is right.

[Meter]

That is wrong. The unit depends on the equation.

In this case.

K = [NH3]2/[N2][H2]3

The unit is (l/mol)².

You do the same thing when working with gases, where the partial pressures are all divided by the standard pressure (1*10⁵ Pa).

Directly from my textbook:

https://prnt.sc/vf4q7u

where c^⦵ = 1 mol/L

[chenbeier]

I understand it. But why I can do so, to devide by 1 mol/ l or 1 bar, if pressure is used. A lot of websides dont consider it. And 40 years ago I went in university i also never heard it.
Its like a price is 100 $ and we devide by 1$ and say its 100. Make this sense? But I have an open ear to learn new things, if they are explained.
Sentences like :"Just because the error is common doesn't mean the idea is right" dont help.
Explanation is given here

https://en.m.wikipedia.org/wiki/Equilibrium_constant