# A tank contains N_2 at 1.0 atm and O_2 at 2.0 atm. Helium is added to this tank until the total pressure is 6.0 atm. What is the partial pressure of the helium?

${p}_{H e}$ $=$ $3.0 \cdot a t m$
We know that the total pressure is $6.0 \cdot a t m$; i.e. $6.0 \cdot a t m$ $=$ ${p}_{{N}_{2}} + {p}_{{O}_{2}} + {p}_{H e}$.
Since we have been given the other partial pressures, calculation of ${p}_{H e}$ is trivial.