Dalton’s Law of Partial Pressures

Dalton’s Law of Partial Pressures

Dalton’s Law of Partial Pressures, named after its discoverer John Dalton, describes the behavior of gas mixtures. It states that the total pressure of the gas, Ptot, is the sum of the partial pressures of each gas.  The partial pressure of a gas is the pressure it would exert if it were alone in the container. In this assignment you will become more familiar with Dalton’s law.

 

1.            Start Virtual ChemLab, select Gas Properties, and then select Dalton’s Law of Partial Pressures from the list of assignments. The lab will open in the Gases laboratory. You will see a gas experiment with eight gas cylinders on the right. Make certain that the switch on the lower right of the lab bench is set to Ideal Gases. Note that the Ideal Gases each have a different molecular weight.

 

2.            Select one of the Ideal Gas cylinders (to the right of the apparatus) by clicking on its white label. Click the red arrow on the brass cylinder valve until the meter reads 400. Add this Ideal Gas to the balloon by clicking the green Open Valve switch once to add gas and again to stop. Add an amount of your choice but do not fill the balloon too full since you will be adding two additional gases. Click Return Tank.

 

3.            Repeat step # 2 for two additional ideal gases of your choice (you don’t have to click on the red arrow for these additional gases though as the valve is already open). Make certain that you do not explode the balloon. If you do, click the Resetbutton located on the upper right of the gas chamber and repeat the experiment (if the Reset button doesn’t work you will need to restart the experiment).

 

4.            Click on the experimental apparatus to Zoom In. There are four LCD controllers on the left for volume, pressure, temperature, and number of moles. You can change the units for volume, pressure, and temperature by clicking on the Units button on each controller. Under pressure and number of moles are numbers 1-8 that correspond to Ideal Gases 1-8. The three gases that you selected will be highlighted. Clicking on each highlighted number will display the partial pressure or moles for that gas alone. Find the Ideal Gas number, the number of moles, and the partial pressure for each of your three Ideal Gases. Record this in the data table.

 

Data Table

 

Ideal Gas Number           Moles (n)            Partial Pressure (Pi)atm

 

5.            Using the information from the data table, determine the total pressure in the balloon (show your working. Give your answer to 3 decimal places)

 

6.            Click Total on the Pressure controller. Compare your answer from # 5 to the total pressure on the meter.

 

Write both pressures below and write a mathematical equation to represent what you have learned about Dalton’s Law.

 

                Total pressure read from pressure controller                     ___________________________

 

Total pressure determined using partial pressure              ___________________________

 

                Mathematical expression for Daltons’ Law               ________________________________

 

7.            Another way of expressing Dalton’s Law of Partial Pressures is with the expression   where Pi is the partial pressure of gas i, xi is the mole fraction of that gas in the gas mixture, and Ptotal is the total pressure. If we had a mixture of two gases A nad B, the mole fraction of gas A would benumber moles of the gas A / ( number of moles of gas + number of moles of gas B)

 

Verify that this relationship  holds using the data you have collected and record you results in the data table below (use the total pressure read from the pressure controller).  Show your working (give all numbers to 2 decimal places)

 

Data Table

 

Ideal Gas Number           xi             Pi (Calculated) atm          Pi (Measured)

 

atm

 

                                               

 

                                               

 

                                               

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8.            Use the following ‘apparatus’ to carry out an experiment that demonstrates Dalton’s Law.  .

 

 

 

https://phet.colorado.edu/sims/html/gas-properties/latest/gas-properties_en.html

 

 

 

Click on ‘explore’ to launch the simulation

 

 

 

 

 

 

 

 

 

 

 

Describe the steps in your experiment, measurements you take and any calculations you do.  You can ‘restart’ the simulation if you need to.  This would count as a step

 

 

 

Format of your answer

 

 

 

1.Write your explanation as a series of steps – step 1, step 2, step 3 etc. 

 

2.  Include the values of any measurements you take on a given step and show any calculations.

 

3.  Keep your description of what you are doing on each step succinct i.e. one or two sentences

 

3. You should be able to validate Dalton’s Law in less than 10 steps

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