Question

A flask contains oxygen, nitrogen, and helium. There are $5moles$ of nitrogen, $4moles$ of helium, and $1mole$ of oxygen. If the total pressure in the flask is $10,000$ torr, then:
I. The partial pressure of helium is $5000torr$
II. The partial pressure of nitrogen is $5000torr$
III. The partial pressure of oxygen is $1000torr$

• A. I, II, and III
• B. I and II only
• C. II and III only
• D. I only

Answer 1

The correct option is C II and III only

According to gas laws, a pressure exerted by a gas is directly proportional to the number of moles of the gas present. We can say $P$ is directly proportional to $n$.

There are $5$ moles of nitrogen, that is $n$ equals to $5$, hence the pressure can be assumed to be $5P$. There are $4$ moles of Helium, so the pressure can be assumed to be $4P$. $1$ mole of oxygen is present, so its pressure can be assumed as $1P$.

The total pressure in the flask is $10,000$ torr.

Dalton's law says that the Total pressure exerted by all the gases in a container is equal to the sum of the partial pressures exerted by an individual gas

Thus, $5P+4P+1P$$=$$10000$

$10P$$=$$10000$

From this, $P$$=$$1000$

Thus, the partial pressure of nitrogen is $5P$ that is $(5\ast 1000)$ that is $5000$ torr.

The partial pressure of oxygen is $1P$ that is $(1\ast 1000)$ that is $1000$ torr.

Option C is the correct answer.