A 118 -mL flask is evacuated, and its mass is measured as...

A 118 -mL flask is evacuated, and its mass is measured as 97.129 g. When the flask is filled with 768 torr of helium gas at $35^{\circ} \mathrm{C},$ it is found to have a mass of $97.171 \mathrm{g}$. Is the gas pure helium?

Key Concepts

Ideal Gas Law

This principle relates the pressure, volume, temperature, and number of moles of a gas through the equation PV = nRT. It is foundational in calculating the amount of gas present under specified conditions and is essential for converting measured pressures, volumes, and temperatures into a corresponding quantity of moles.

Molar Mass Determination

This concept involves finding the mass of one mole of a substance. By measuring the mass change when a gas is introduced into a container and using the Ideal Gas Law to compute the number of moles present, one can calculate the gas's molar mass, which helps in identifying or confirming its identity.

Gas Purity Assessment

This involves analyzing the calculated molar mass of a gas sample and comparing it with the expected molar mass of a pure gas. If there is a significant discrepancy, it indicates possible contamination or the presence of other gases, making this analysis a crucial step in verifying gas purity.

Transcript

00:04 In this question, we have a 118 milliliter flask that is evacuated and measured at 97 .129 grams.

00:13 And then the flask is filled with 768 tor and 35 degrees celsius of helium, and now it has a mass of 97 .171 grams.

00:22 And they're asking, is the gas pure helium? so what we're going to do is use the information they've given us to find the molar mass of the gas in the flask.

00:34 Compare that to the molar mass of helium.

00:37 So what i'm going to do is use my conditions here to find the number of moles of gas that are in the flask.

00:47 And then we'll use these numbers down here, the masses to get a grams per mole of molar mass.

00:54 So to find the number of moles in the flask, i'm going to use the ideal gas equation, which is pv equals n -r -t.

01:01 P is pressure, v is volume, n is number of moles.

01:05 That's what we're looking for here.

01:07 R is a constant and t is temperature.

01:09 But first i have to put all of my numbers into the correct units to use this equation.

01:16 So my volume needs to be in liters.

01:19 So to get from mill liters to liters, i will divide by a thousand.

01:23 So i have 0 .118 liters.

01:26 My pressure needs to be in atmospheres.