Working with the Ideal Gas Law
Prelab
NOTE: AT THIS POINT YOU WILL ANSWER ALL PRELAB QUESTIONS IN YOUR
CARBON COPY LAB NOTEBOOK. BE SURE TO INCLUDE THE NAME OF THE
EXPERIMENT AND THE QUESTIONS ASKED. AT THE START OF LAB YOU WILL
HAND IN THE CARBON COPY, WHICH WILL CONTAIN ANSWERS TO ALL
QUESTIONS EXCEPT QUESTION 3 WHICH SHOULD BE ON A SEPARATE PAGE
SINCE YOU WILL NEED THIS FOR THE LAB SESSION.
1. What is the purpose of this experiment?
2. Show the calculation for the mass of NaNO2 required to produce 40. mL of N2 with a
temperature of 25.6oC and an atmospheric pressure of 739 mmHg. (use equation (2))
3. Copy the results pages in your carbon copy notebook.
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Working with the Ideal Gas Law
This experiment will enable you to collect a gas (N2) evolved in a given reaction and measure its
temperature, volume and pressure. Assuming this is an ideal gas, the number of moles of
nitrogen formed in this reaction can be calculated using the ideal gas equation. The amount of
sodium nitrite, NaNO2, in a sample will be determined by measuring the amount of nitrogen gas
evolved in a chemical reaction.
Introduction
Nitrogen gas (N2) is colorless and shows a great reluctance to react with other elements or
compounds. This gas is a very useful material. The largest quantity of this gas is used to
provide a nonoxidizing environment for packaged foods and wine. Nitrogen gas is also used to
pressurize electric cables and telephone wires. The major use of this gas is in the production of
fertilizers. Nitrogen occurs in protein and nucleic acids. Nitrogen constitutes about 3% of the
human body. The nitrogen content of these biological materials is determined by adding excess
nitrous acid (HNO2) and a strong acid to the materials and measuring the amount of nitrogen gas
evolved. In this experiment, an inorganic compound, sulfamic acid will be used as a nitrogen
source instead of biological material.
When sodium nitrite, NaNO2, is dissolved in water, it dissociates into ions (Na+, NO2-). Nitrous
acid is a rather weak acid and when the sodium nitrite solution is combined with a solution of a
strong acid like sulfamic acid, HNH2SO3, almost all NO2- is converted to HNO2 and the
following reaction occurs releasing nitrogen gas:
HNO2 (aq) + NH2SO3- (aq) → N2 (g) ↑ + HSO4- (aq) + H2O (l)
(1)
Part of the nitrogen comes from HNO2 and part from NH2SO3-. In this experiment, NH2SO3- is
present in excess, so each mole of NaNO2 you start with should lead to one mole of N2 gas.
Observe the reaction you will be working with in this experiment.
NaNO2 (aq) + HNH2SO3 (aq) → N2 (g) ↑ + NaHSO4 (aq) + H2O (l)
(2)
In Part I of this experiment, you will test this stoichiometric relationship from equation (2) with a
known amount of sodium nitrite. In Part II, you will again use this relationship to determine the
percentage of sodium nitrite in an unknown mixture. In order to calculate this percentage for
Part II, you will first have to determine how many moles of nitrogen gas are produced in the
reaction. We are going to assume ideality and use the following equation:
n N2 =
(Patm − PH 2O )Vb
RT
where the value of the constant R is 82.05 mL•atm/mole•K and Vb, is the volume change in the
buret. Patm is the atmospheric pressure and PH2O is the vapor pressure of water at a certain
temperature. A table of these vapor pressures at different temperatures are at the end of this
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laboratory experiment. Be sure to estimate between temperatures when necessary. The
temperature for this experiment will be the room temperature of the laboratory. Remember,
when converting from mmHg to atm, the conversion is 1 atm = 760 mmHg.
Procedure
Part I
1. Using the ideal gas law equation, calculate the amount of NaNO2 required to produce 40 mL
of nitrogen gas (N2) at the room temperature and pressure according to equation (2). Show
this calculation on your calculations page.
2. Clean, dry and weigh a small test tube. Weigh approximately the amount of NaNO2 you just
calculated in step 1. Be accurate to 0.1 mg. Do not weigh out more than you need, if
this happens, remove some of the NaNO2 and reweigh.
3. Dissolve the NaNO2 by adding 2 mL of distilled water to the small test tube. When the
NaNO2 is dissolved, place this test tube in a marked beaker to prevent it from overturning
until you are ready to place it in the larger tube.
4. Fill a large beaker of distilled water and let it sit on your bench until it has reached room
temperature (to within a degree). Be sure to record the room temperature since you will need
this temperature to determine the vapor pressure of water.
5. Your apparatus for this experiment has already been set up for you and your instructor will
inform you of the names of the different parts of the apparatus. Check that all of the
connections on the apparatus are snug so there is no leakage and that the rubber tubing isn’t
pinched or folded shut.
6. You have to stabilize the pressures both inside and outside the system. This means both
pressures must be the same. First, open the system by removing the rubber stopper from the
large test tube. Support the leveling bulb at a level near the top of the buret and pour the
room temperature water into the buret until it is nearly full. At this point, insert the rubber
stopper back into the large test tube. By changing the height of the leveling bulb, you are
changing the total pressure within the apparatus. When the water levels in the leveling the
bulb and buret are identical, the pressures are identical both inside and outside the system.
7. To test for leaks, be sure all the stoppers are in place and raise the leveling bulb to maximum
height. This increases the pressure inside the apparatus. Watch the water level inside the
buret for a minute or so. If there is no change, then your apparatus is airtight. If the level
rises slowly, there is a leak and you must find it. If the level stops rising when you pinch the
rubber tubing connecting the large test tube to the buret, you have found the source of your
leak. If the level continues to rise, the leak is in a connection near the test tube. When your
apparatus is working correctly, return the leveling bulb to a level near the top of the buret.
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8. Remove the large test tube from your apparatus. Weigh out 0.5g of sulfamic acid and place it
in the large test tube. You do not need to be exact. Add 10 mL of distilled water to the large
test tube and swirl to dissolve.
9. Carefully, lower the small test tube containing the nitrite sample into the large test tube. Do
not allow the nitrite and the acid to come in contact.
10. Reconnect the large test tube to the apparatus and place it in the 1-litre beaker. At this time,
carefully pour the room temperature water into the beaker until the level is above the liquid
level inside the large test tube.
11. Recheck your apparatus for leaks. Next, repeat step 6 and have both liquid levels in the
buret and leveling bulb identical. Make sure that when the levels are identical, the level is
near the top of the buret. Keep the bulb and buret at this same level until the start of the
experiment.
12. When everything is at the proper condition, place the rubber stopper back in the large test
tube.
13. Record the initial volume in the buret. Remember that the large markings on the buret
correspond to millilitres and the smaller markings to 0.1 millilitres. You initial volume
should be recorded to the nearest 0.01 mL
14. One student should be keeping the levels in the buret and leveling bulb constant throughout
step 15 while the other student performs step 15.
15. Lift the large test tube from the water bath and tip it so that a little of the nitrite solution
flows out of the small tube into the sulfamic acid solution. Bubbling should occur. Return
the large test tube to the water bath and shake it gently until the bubbling stops. Repeat this
step until all of the nitrite solution has mixed with the acid. Keep shaking gently to insure
good mixing. Avoid letting the solution from the test tube flow into the buret.
16. When the solutions from the small and large test tube mix, the gas is evolved and the liquid
in the buret will drop quickly. Be sure to keep the two liquid levels about the same
throughout the entire reaction.
17. When the reaction is complete, make sure the bath temperature water has remained the same.
If not, make the proper adjustments. Adjust the leveling bulb to make the liquid levels
identical and record the final volume reading on the buret.
18. Repeat this procedure one more time with another sample of sodium nitrite.
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Part II
1. Obtain a packet containing NaNO2 and a filler. You must determine the percent by mass of
sodium nitrite in this mixture. Be sure to record the number for your unknown.
2. Weigh out about the same mass in your small test tube as you did in Part I. Fold your packet
when you are finished weighing so that your unknown doesn’t absorb any moisture.
3. Repeat the same procedure that you performed in Part I. Do this twice.
4. If you find there is inconsistency in your results, you may want to perform a third run and
disregard the most inconsistent run. Should you do this, explain your reasons for doing so.
5. Hand in all of your carbon copies from this lab at the end of class.
Vapor Pressure of Water at Different Temperatures
Temperature (oC)
10
15
16
17
18
19
20
21
22
23
Temperature (oC)
Vapor Pressure (mmHg)
9.2
12.8
13.6
14.5
15.5
16.5
17.5
18.6
19.8
21.1
24
25
26
27
28
29
30
35
40
45
39
Vapor Pressure (mmHg)
22.4
23.8
25.2
26.7
28.3
30.0
31.8
42.2
55.3
71.9
Results
Part I
Write the chemical reaction you will be studying in this experiment in the space below.
Atmospheric pressure _______ Room temperature _______
Water vapor pressure _________
Trial 1
Trial 2
Mass of small test tube and NaNO2
___________
___________
Mass of empty small test tube
___________
___________
Mass of NaNO2
___________
___________
Moles of NaNO2
___________
___________
Theoretical yield of moles of N2 (eq (2))
___________
___________
Final Buret reading
___________
___________
Initial Buret reading
___________
___________
Volume change (Vb)
___________
___________
Patm - PH2O
________________
________________
Moles of N2 gas produced experimentally
___________
___________
% yield of N2
___________
___________
Average % yield of N2
___________
Show a sample calculation for everything in your notebook. Include all conversions and units.
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Part II
Write the chemical reaction you will be studying in this experiment in the space below.
Atmospheric pressure _______ Room temperature _______
Unknown # ___________________
Trial 1
Water vapor pressure _________
Trial 2
Mass of small test tube and unknown sample___________
___________
Mass of empty small test tube
___________
___________
Mass of unknown sample
___________
___________
Final Buret reading
___________
___________
Initial Buret reading
___________
___________
Volume change (Vb)
___________
___________
Patm - PH2O
________________
________________
Moles of N2 gas produced
___________
___________
Moles of NaNO2 in unknown sample
(assume 100% yield of N2)
___________
___________
Mass of NaNO2 in unknown sample
___________
___________
% NaNO2 (by mass) in unknown sample
___________
___________
Average % NaNO2 in unknown sample
____________
Show a sample calculation for everything in your notebook. Include all conversions and units.
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