Gas Laws and Molar Mass Calculation

Choose ALL of the answers that are units of pressure

• K

• M

• kPa

• atm

• mm Hg

Answer explanation

The units of pressure are atm (atmospheres), mm Hg (millimeters of mercury), and kPa (kilopascals), so these are the correct choices.

What is -5 ^oC in Kelvin?

• 268

• 278

• 100

• 5

Answer explanation

To convert Celsius to Kelvin, add 273.15. Therefore, -5°C + 273.15 = 268K. Hence, -5°C is equal to 268 Kelvin.

Which law is used to find the moles of gas in a sample?

• Charles Law

• Boyle's Law

• Combined Gas Law

• Ideal Gas Law

Answer explanation

The Ideal Gas Law is used to find the moles of gas in a sample when you know the pressure, volume and temperature.

Which diagram best shows the end result of diffusion?

• F

• G

• H

• J

Answer explanation

Diagram J best shows the end result of diffusion as it depicts the spreading of particles from an area of high concentration to an area of low concentration.

The picture is an example of _________________.

• active transport

• diffusion

• osmosis

• effusion

Answer explanation

The picture is an example of effusion, which is the process of a gas escaping through a small opening into a vacuum or low-pressure area.

Diffusion is the movement of molecules from an area of _____ concentration to an area of ______ concentration

• High, low

• Low, high

• Low, low,

• High, high

Answer explanation

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration, making 'High, low' the correct choice.

Find the density of Neon gas at .725 atm and 25°C

• .35 g/L

• .47 g/L

• .60 g/L

• .79 g/L

Answer explanation

To find the density of Neon gas, use the ideal gas law formula: density = (pressure * molar mass) / (gas constant * temperature). Plugging in the values, density = (.725 * 20.18) / (0.0821 * 298) = 0.60 g/L. Make sure to use the value of the Ideal Gas Constant, R, that matches the units of the pressure (in atm)

At 250. K and 1.75 atm, a gas has a mass of 15.3 g and occupies 5.60 L. What is the molar mass of the gas?

• 32.04 g/mol

• 42.15 g/mol

• 48.91 g/mol

• 54.28 g/mol

Answer explanation

To find the molar mass, use the ideal gas law formula PV = nRT to calculate moles, then divide the mass of the gas given in the problem by the moles you just calculated to get the molar mass. The correct molar mass is 32.04 g/mol.

An unknown gas diffuses 0.25 times that of Helium. What is the molar mass of the unknown gas?

• 1 g/mol

• 64 g/mol

• 24 g /mol

• 16 g/mol

Answer explanation

The molar mass of the unknown gas can be calculated using Graham's law of diffusion. Rate of the unknown gas divided by the Rate of Helium = 0.25. This equals the square root of the inverse of the molar masses. So 0.25 = Square root of (Molar Mass of Helium divided by the Molar Mass of the unknown) Since the unknown gas diffuses 0.25 times faster than Helium, the molar mass of the unknown gas is 4 times that of Helium, which is 4 * 16 g/mol = 64 g/mol.

What is the rate of diffusion of Oxygen and Ammonia gas (NH₃)? Which diffuses faster?

• Ammonia 1.37 times faster than Oxygen

• Oxygen 1.37 times faster than Ammonia

• Ammonia 0.83 times faster than Oxygen

• Oxygen 3 times faster than Ammonia

Answer explanation

Ammonia diffuses 1.37 times faster than Oxygen, based on the rate of diffusion of gases. The lighter gas (the one with the smaller molar mass) diffuses faster. To determine how much faster use Rate Ammonia/Rate Oxygen = square root of (Molar Mass of Oxygen divided by Molar Mass of Ammonia) Remember that Oxygen is Diatomic so it's Molar Mass is 16 * 2 = 32 g/mol