A mixture of helium, nitrogen and oxygen has a total pressure of 781 mmHg. The partial pressure of helium is 331 mmHg, and the partial pressure of nitrogen is 222 mmHg. What is the partial pressure (in mmHg) of oxygen in the mixture?

Partial pressure of O₂ =  228 mmHg

Explanation:

The mixture is made of three gases, He, N₂ and O₂.

Total pressure of it, is 781 mmHg.

There is a law from gases, that indicates the sum of partial pressures of each gas in a mixture will be the total pressure of the system. In conclussion:

Partial pressure of He + Partial pressure of N₂ + Partial pressure of O₂ = Total pressure.

331 mmHg + 222 mmHg + Partial pressure of O₂ = 781 mmHg.

Partial pressure of O₂ = 781 mmHg - 331 mmHg - 222 mmHg

Partial pressure of O₂ =  228 mmHg

im not exactly sure but i think its b!