Intro Lesson: Collecting Gas Over Water

​Dalton's Law of Partial Pressure and Collecting Gas Over Water

The total pressure of a mixture of gases is equal to the sum of all of the gases combined

​

Dalton's Law of Partial Pressure

Drag and Drop

A container contains a mixture of several gases including nitrogen, oxygen, and carbon dioxide. The pressure of the nitrogen is 2.00 atm, the pressure of the oxygen is 2.50 atm, the pressure of carbon dioxide is 1.25 atm. The total pressure is ​

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. I know this because ​

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states that ​

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is equal to the ​

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.

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5.75 atm

Dalton's Law of Partial Pressure

the total pressure in a mixture of gases

sum of the individual pressures

6.25

ideal gas law

it is an equation of state

product of the individual pressures 

Drag and Drop

A mixture of gases has a total pressure of 10.55 atm. The mixture contains hydrogen gas at a pressure of 2.75 atm, nitrogen gas at 5.00 atm, and oxygen gas at pressure of ​

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. I know this because all of the pressures should​

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to equal the ​

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pressure. This is an example of ​

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of partial pressure.

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2.80 atm

add up

total

incomplete 

18.30 atm

Dalton's Law

• When a liquid, such as water, is sitting in a closed container some of the liquid evaporates into a gas.

• The gas above the liquid exerts a "pressure" on the container.

• This pressure is called "vapor pressure".

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What is Vapor Pressure (Pvap)

• Notice in the chart to the left that as the temperature of the water increases the vapor pressure of the water increases.

• This makes sense because at higher temperatures more water evaporates)

The of Vapor Pressure of water is dependent upon the temperature of the water

Drag and Drop

Vapor pressure is caused by the ​

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of a liquid in a ​

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. The vapor that is ​

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collides with the ​

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to cause a ​

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and this pressure is referred to as the "vapor pressure".

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evaporation

closed container

open container

above the water

in the water

walls of the container

surface of the water

pressure

condensation

Drag and Drop

As the temperature of water in a closed system increases the amount of liquid that evaporates ​

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causing the vapor pressure to​

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. Thus, the vapor pressure is dependent upon the ​

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of the water.

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increases

go up

do down 

decreases

temperature

volume

• What you see to the far left is a picture of an inverted graduated cylinder full of water.

• Notice, as you move from left to right, the volume of the water is decreasing.

• This is due to a gas being pumped into the system through the brown tube

• The new gas displaces the water and pushes it down.

• The area on top of the water now contains the gas produced AND some water vapor (a mixture of gases!!!)

​

​Collecting Gas Over Water

Drag and Drop

A student runs an experiment that produces a gas. The student collect the gas OVER WATER as seen in the picture to the left. The area above the water contains ​

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. The types of gases present above the water are the gas collected and ​

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. Because there are two gases present we have a ​

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and the total pressure will be equal to the ​

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of the pressures of the two individual ​

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.

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two types of gases

water vapor

mixture of gases

sum

gases

product 

liquid

empty space 

air

Drag and Drop

A student collected gas over water The vapor pressure of the water is dependent upon the ​

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of the water. The two gas above the water consists of the gas collected AND ​ ​

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. Thus, we have a ​

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problem. The temperature of the water is 25 degrees Celsius and the Pvap of water at this temperature is 24 mmHg. The total pressure of the gas above the water is found to be 759 mmHg. Therefore, the pressure of the gas collected must be ​

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. I know this because the two pressures must add up to be ​

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temperature

volume 

water vapor

735 mmHg

partial pressure

759 mmHG

783 mmHg

• Atmospheric pressure is caused by the atmosphere pushing down on

  you or a container.

• ​The total pressure inside a gas collected over water is EQUAL to the atmospheric pressure.

• The pressure of the gas collected is equal to the atmospheric pressure minus the vapor pressure.

​

Atmospheric Pressure
and collecting a gas
over water

Drag and Drop

A student collects a gas over water. The atmospheric pressure is 760 mmHg and the temperature of the water is 20 degrees Celsius. The Pvap of water at this temperature is roughly 18 mmHg. If the atmospheric pressure is 760mmHg then the total pressure inside the container is ​

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. Therefore, the pressure of the gas collected will be ​

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. I know this because ​

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states that the ​

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of a mixture of gases is equal to the ​

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of the pressure of the ​individual gases present.

Drag these tiles and drop them in the correct blank above

742 mmHg

778 mmgh

Dalton's Law of Partial Pressure

the ideal gas law 

total pressure

sum

difference

760 mmHg

Drag and Drop

Task: A certain experiment generates 2.58 L of hydrogen gas which is collected over water. The temperature is 20 C and the atmospheric pressure is 98.60 kPa.

Step 1: Find the volume of the dry (not over water) hydrogen gas at STP. At 20 C the Pvap of water is 2.38 kPa. Therefore, the pressure of the hydrogen gas collected is​

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.

Step 2: I now know P1, V1 and T1 for my hydrogen gas. P2 and T2 will be standard, therefore P2 = ​

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and T2 = ​

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. Step 3: Use the combined gas law to determine the volume (V2) of the dry hydrogen at STP .

Plug in your answer on the next slide.

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96.22 kPa

273k

101.325 kPa

Math Response

The answer to the previous question is: ____ Liters (your answer should go to the hundredth's place).

Type answer here

Correct Answer: 2.28

Deg^°

Rad