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.

P air P 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 kPa This is often called the pressure of the “dry” gas. Table 18.2 Page 459

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).

242cm 3 of a gas is collected at 87.6kPa. Find the volume at standard pressure.

Problem from student’s notes 31.5cm 3 collected at 97.8kPa. What pressure would produce a volume of 37.4cm 3 ?

Alternative version 31.5cm 3 collected at 97.8kPa. What pressure would produce a volume of 37.4cm 3.

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 1.0 atm pressure. If the volume of the cylinder is 5.0 L, what is the pressure of the gas inside the can?

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 m 3 ) is 760 mmHg before the storm. At the peak of the storm, the pressure outside the classroom drops to 596 mmHg. a. To what volume would the air in the room try to change to so as to equalize the pressure?

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

Lung Function and Boyles Law

When you inhale, the diaphragm and intercostal muscles (those are the muscles between your ribs) contract and expand the chest cavity. This expansion lowers the pressure in the chest cavity below the outside air pressure. Air then flows in through the airways (from high pressure to low pressure) and inflates the lungs. When you exhale, the diaphragm and intercostal muscles relax and the chest cavity gets smaller. The decrease in volume of the cavity increases the pressure in the chest cavity above the outside air pressure. Air from the lungs (high pressure) then flows out of the airways to the outside air (low pressure). The cycle then repeats with each breath.

Homework Worksheet: Boyle’s Law and Collecting a Gas Over Water