5.2.3 Applications of the Gas Laws

Question 1

Applications of the Gas Laws

Answer 1

• When determining the number of moles of gas collected over water, the partial pressure of water vapor in the gas sample must be taken into account.
• Helium balloons and hot air balloons float because the gas contained within them has a lower density than the atmosphere.

Question 2

note

Answer 2

• Collecting a gas over water is a technique that allows gas produced from a reaction to be collected without mixing it with air.
• When determining the number of moles of gas collected over water, the partial pressure of water vapor in the gas sample must be taken into account.
• Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g)
• Mg metal is reacted with HCl and the gas produced is
  collected over water. If 36.0 mL of gas are collected at 20.0 ̊C and 765.0 torr total pressure, how many moles of H 2 are formed? The vapor pressure of water at 20 ̊C is 17.5 torr.
• First, use Dalton’s law of partial pressures to calculate the partial pressure of H 2 .
• Next, use the ideal gas law (PV = nRT) to calculate moles of H 2 . The pressure must be converted to atmospheres and the temperature to kelvins. Solving for n yields 1.47 x 10^–3 mol H 2 .
• Why do helium balloons float at room temperature?
• Less dense substances float over more dense substances.
• 1.00 mol of any gas occupies 24.4 L at 1.00 atm and 298 K (room temperature).
• However, 1.00 mol He weighs 4.00 g, while 1.00 mol N 2 (the major component of air) weighs 28.0 g.
• The density of helium is lower than the density of nitrogen, so helium balloons float.
• How do hot air balloons work?
• 1.00 mol of gas at 1.00 atm and 600 K occupies 49.2 L, while 1.00 mol of gas at 1.00 atm and 298 K occupies 24.4 L.
• This means that the density of hot air inside a balloon (0.569
  g/L) is lower than the density of the surrounding air (1.15
  g/L). This causes hot air balloons to float.

Question 3

What is the correct equation for converting pressure from units of torr to units of atmospheres (atm)?

Answer 3

760.0 torr = 1 atm.

Question 4

What is the density of methane gas, CH4, at STP?

Answer 4

0.714 g / L

Question 5

Which of the following statements best describes the vapor pressure of water?

Answer 5

• It depends on the temperature of the water only.

- It is always a component of the total pressure when a gas is collected over water.

Question 6

Is the following statement true or false?

The gas sample collected when collecting hydrogen over water is pure hydrogen gas.

Answer 6

false

Question 7

A sample of hydrogen gas is collected over water. We measure the temperature, pressure, and volume, and use our knowledge of the gas law to calculate that the sample contains 1.43 × 10^−3 moles of hydrogen. What is the mass of the hydrogen collected?

Answer 7

2.88 mg

Question 8

A sample of H2 gas is collected over water at 20.0°C from the reaction
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g).
The pressure of gas in the flask is 770.0 torr. What is the pressure of “dry hydrogen” in PH2O = 17.5 torr at 20.0 C the flask?

Answer 8

752.5 torr

Question 9

On a nice summer day on a mountain top, the temperature is 29.0°C and the air pressure is 700.0 torr. What is the volume of 1.0 mole of gas? (R = 0.08206 L • atm / mol • K)

Answer 9

27 L

Question 10

At a factory near the coast (at sea level, where air pressure is 760 torr), 1.00 mole of chlorine gas, Cl2 is pumped into a steel tank in a large walk-in refrigerator, at 4.0°C. What is the density of the chlorine gas?

Answer 10

3.1 g / L

Question 11

In the lecture, Professor Yee discussed a chemical reaction between magnesium metal (Mg) and hydrochloric acid (HCl). What was illustrated with the use of this reaction?

Answer 11

to illustrate the collection of a gas over water

Question 12

Identify the staement which best explains why a hot air balloon rises.

Answer 12

The density of hot air is less than the density of cold air.