When a 118-mL flask is evacuated, it has a mass of 97.129 g. Upon filling it with 768 torr of helium gas at 35 °C, the mass increases to 97.171 g. Based on this information, can we determine if the helium gas was pure?

Verified step by step guidance

Calculate the mass of the helium gas by subtracting the mass of the evacuated flask from the mass of the flask with helium.

Convert the pressure from torr to atm using the conversion factor: 1 atm = 760 torr.

Convert the temperature from Celsius to Kelvin by adding 273.15 to the Celsius temperature.

Use the ideal gas law, PV = nRT, to calculate the number of moles of helium gas. Use the mass of helium, the molar mass of helium (4.00 g/mol), and the number of moles to find the molar mass.

Compare the calculated molar mass with the known molar mass of pure helium to determine if the gas is pure.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ideal Gas Law

The Ideal Gas Law relates the pressure, volume, temperature, and number of moles of a gas through the equation PV = nRT. This law allows us to calculate the expected mass of a gas under specific conditions, which can be compared to the measured mass to assess purity.

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). For helium, the molar mass is approximately 4.00 g/mol. By calculating the expected mass of helium in the flask using its molar mass, we can determine if the measured mass aligns with pure helium.

Mass Change and Density

The change in mass of the flask before and after filling it with gas indicates the mass of the gas added. By calculating the density of the gas using the mass and volume, we can compare it to the known density of pure helium (0.1786 g/L) to evaluate the purity of the gas in the flask.

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Textbook Question

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