# Calculate the partial pressure of NO2

Calculate the partial pressure of $\ce{NO2}$.

$$p_{\ce{N2}}/p_{\ce{O2}}=5$$

$$\ce{N2(g) + O2(g) <=> 2NO(g)}$$

Let’s assume $K=0.1$

$$K= \frac{p_{\ce{NO}}^2}{p_{\ce{O2}}\cdot p_{\ce{N2}}}$$

$$K= \frac{p_{\ce{NO}}^2}{p_{\ce{O2}}\cdot p_{\ce{O2}}\cdot5}$$

But how do I proceed?

Ok. Now you have 3 variables and 2 equations. To solve the problem you need 3 equations.

Third equation you'll get from the partial pressure law:

The total pressure of an ideal gas mixture is the sum of the partial pressures of each individual gas in the mixture.

Tip: suppose total pressure is 1.

• so? p(o2)+p(N2) =1 Aha so I can choose whatever sum I want? – MrGuest Oct 22 '15 at 10:37
• Don't forget about NO. – Alex Oct 23 '15 at 9:23

When applying partial pressures please do not forget the partial pressure that the product itself exerts (NO)

So we have: p(O2)+ p(N2) + p(NO) = total pressure of the system.