# Equilibrium expressions and partial pressures

## Homework Statement

In considering the reaction:
3NO (g) + H2O (l) → 2HNO3 (aq) + NO (g)
student A writes the equilibrium expression as:
K=[NO][HNO3]2/[NO2]3
while student B writes:
K=PNO [HNO3]2/[NO2]3

Whose equation is correct?

NA

## The Attempt at a Solution

This is a multiple choice problem but I wanted to know how each student would arrive at a different value for K.

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Borek
Mentor
I suppose you mean

3NO2(g) + H2O ↔ 2HNO3 + NO(g)

Lousy question IMHO, as both equations can be used this way or another. I prefer the first, as it is consistent, the latter would be OK with me if written as

$$K = \frac{P_{NO}[HNO_3]}{P_{NO_2}^3}$$

Don't worry about the fact K values are different. That's normal, K value depends on the way reaction equation is written. As long as you are consistent you will get the same final result (and if you know the reaction equations converting between different values of K is not that difficult).

Thank you!