Chapter 18

Collecting a Gas Over Water

&

Boyle’s Law

Collecting a Gas Over Water

The gas is collected and the pressure inside the flask is equalized with the external air pressure.

PairPgas

If the external pressure is 99.8 kPa and the temperature is 25°C what is the pressure of the hydrogen gas?

If the external pressure is 99.8 kPa and the temperature is 25°C what is the pressure of the hydrogen gas?

99.8 kPa – 3.2 kPa = 96.6 kPaThis is often called the pressure of the “dry” gas.

Table 18.2 Page 459

If the external pressure is 99.8 kPa and the temperature is 25°C what is the pressure of the hydrogen gas?

Boyle’s Law

Boyle’s Law

• The pressure and the volume of a gas are inversely related.

Even when they have an ample supply of oxygen gas, airplane passengers become uncomfortable if the cabin loses

its pressure. (Hint: What will the gases in the sinuses and inner ear do if exposed to a sudden drop in air pressure).

Tennis balls are sold in pressurized cans (Hint: Each tennis ball contains gases at elevated

pressure to give it good bounce).

242cm3 of a gas is collected at 87.6kPa.Find the volume at standard pressure.

= 209 cm3

Initial

242cm3

87.6 kPa

Final

→ ?

→ 101.3 kPa

242cm3

P↑ V↓

x 87.6 kPa101.3 kPa

31.5cm3 collected at 97.8kPa. What pressure would produce a volume of 37.4cm3.

82.4 kPa

You buy a helium balloon kit to inflate party balloons. Assume that the helium cylinder indicates that it contains about 252 liters of helium. This volume refers to the volume of helium at 1.00 atm pressure. If the volume

of the cylinder is 5.00 L, what is the pressure of the gas inside the can?

50.4 atm

Tornados

• A tornado is a vortex of rapidly moving air associated with some severe thunderstorms. Winds within the tornado funnel may exceed 500 kilometers per hour. High velocity winds cause most of the damage associated with these weather events. Tornadoes also cause damage through air pressure reductions. The air pressure at the center of a tornado averages about 600 torr and many human made structures collapse outward when subject to pressure drops of this magnitude.

Suppose that on a stormy spring afternoon, a tornado passes by Tri-Valley High School. The air pressure inside the

classroom (volume = 430 m3) is 760 mmHg before the storm. At the peak of the storm, the pressure outside the

classroom drops to 596 mmHg. To what volume would the air in the room try to change to so as to equalize the pressure?

548 m3

Why is it a good idea to open windows slightly as such a storm approaches?

Lung Function and Boyles Law

Homework

• Worksheet: Boyle’s Law and Collecting a Gas Over Water– Do your work on the back of this worksheet or

on a separate sheet of paper.– You will need the table of water vapor

pressures in your notebook to answer some of the questions.