Gas Laws

Several brands of antacid contain calcium carbonate as their active ingredient. Inert materials such as binders, coloring, and flavorings are also present. In this experiment you will determine the weight percent of calcium carbonate in an unknown antacid tablet.

Calcium carbonate reacts with hydrochloric acid and produces carbon dioxide, as shown in the following equation:

By reacting the antacid with excess acid and then collecting the carbon dioxide produced in a constant predetermined volume, we will be able to use the ideal gas law and the molecular weight of calcium carbonate (100.0869 g/mol) to determine the weight percent of calcium carbonate in the antacid tablet.

The pressure of the flask will be monitored with a pressure sensor and temperature will be monitored using a temperature probe, both connected to a Vernier computer interface unit. Using the ideal gas law and a known standard sample of calcium carbonate you will determine the volume of the flask.

Apparatus and Chemicals

Apparatus

Chemicals

Procedure

Preparation of the Apparatus

Vernier ID

Each Vernier interface is identified by a number in the top right corner. Make sure to mark this number on the data sheet so that we can identify any problems with the experimental setup.

The instrument should be ready to use, and the display should look like Figure 1.

If it does not, please inform your TA and they will help you identify the problem. The instrument is used in monitor mode only, so you will not need to access the touch screen once it is set up. Verify that the pressure sensor is connected to Channel 1 and the temperature probe to Channel 2 as in Figure 1. Clearly label your 125 mL Erlenmeyer flask.

Volume determination with standard CaCO₃

Place your flask on the balance and press O/T.

Wet Glassware

It is not crucial for your glassware to be dry. Calcium carbonate does not dissolve appreciably in water, so some residual water should have no impact on your experiment.

Practice

It is highly recommended that you practice the following steps with an empty vial and your empty 125 mL Erlenmeyer flask before actually doing the steps in the experiment.

Use a dry spatula to transfer about 0.4 g of standard $\ce{CaCO3}$ into the flask. Record the precise weight in your datasheet. Tap the flask gently to move the sample to the side of the flask. If you use more than 0.5 g of $\ce{CaCO3}$, too much $\ce{CO2}$ will be produced; the pressure will exceed the capacity of the setup and that run will be unusable. However, using too small of a sample will result in a small pressure change, which will decrease the precision of the run.
Fill a clean vial 3/4 full (~6 mL) with 4.5 M $\ce{HCl}$. Wipe any spilled acid from the outside of the vial.
Grip the vial of acid with your tweezers at one edge of the vial near its top.
Carefully lower the vial, setting it upright in the bottom of the flask without touching the sample of calcium carbonate.
Carefully clamp the flask at the neck to the bench stand so that it will not fall over.
Open the 2-way valve and firmly press the rubber stopper into the flask. Your setup should now look like that shown in Figure 2.
Connect the vacuum tubing to the 2-way valve and ensure the valve is closed.
Open the vacuum tap, and then open the 2-way valve. Pressure in the flask will decrease.
When the pressure gets between 15 - 30 kPa, close the 2-way valve. Disconnect the vacuum tubing, and then turn off the vacuum tap.
Wait until the pressure is stable (i.e. pressure does not change by more than 0.1 kPa for 5 seconds).

assembly before the reaction — Assembly before reaction

Leaks

If the system is not tightly sealed, the pressure will not stabilize. You must find and seal any leaks before going further. Most of the time, the leak is found around the rubber stopper. Press the stopper down firmly to seal the gap between the flask and stopper. If you cannot find and seal your leak, consult your TA.

Reaction

Record the initial pressure and temperature in the datasheet.
Remove the flask from the extension clamp. Hold the flask gently by the neck and agitate it until the vial falls over and the acid spills into the flask.
Shake the flask continuously for about 2 minutes to ensure complete reaction of the $\ce{CaCO3}$. As the acid reacts with the antacid, $\ce{CO2}$ is rapidly released and the pressure in the flask will increase. Stop shaking when the gas ceases to be produced (the pressure stops increasing and remains stable). The setup at this point should look like Figure 3.

Record the final temperature and pressure in your datasheet.
Open the 2-way valve and remove the rubber stopper from the flask.
Wash the flask for the next run.
Repeat the procedure twice with similar amounts of $\ce{CaCO3}$.

Running the experiment with the antacid tablets

Make sure your flask is clean and still clearly labeled before you start. Precisely weigh the antacid tablet (use a weighing boat); record the weight and code number on your datasheet. Cover the tablet with a piece of Kimwipe towel and crush it into small pieces. You will do 3 runs with the antacid. Proceed exactly as for the standard $\ce{CaCO3}$ procedure using about one quarter to one third of a tablet for each run. Record all measurements in your datasheet. Remember to record not only to mass of the entire tablet, but also the mass of the portion of the tablet that you use for each trial.

Pre-Lab

Write out the procedure in your own words in point form. You do not need to include procedural elements related to the use of the Vernier interface. You may write it out by hand or type it and print it.

Clean-up

Wash your flask and vial thoroughly with tap water (use soap and brush if necessary). When they are clean, rinse thoroughly with tap water, then once with distilled water. Finally, pour about 25 mL of ethanol in the Erlenmeyer and swirl. Transfer the ethanol to the vial and then discard down the sink.

Lab Grade Breakdown

Please be aware that you will be graded on the accuracy and precision of your final result.

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Calculations

The ideal gas law relates the number of moles $n$ of a gas with the pressure $P$ it creates in a volume $V$ at temperature $T$.

The initial pressure $P_i$ measured in the flask before the reaction (Figure 2) is the result of the contributions of Air and $\ce{H2O}$ gases:

After the reaction (Figure 3), Air, $\ce{H2O}$ and $\ce{CO2}$ contribute to the pressure.

$\ce{H2O}$ in the flask is present both in the liquid phase and in the gas phase. How much will be in the gas phase versus liquid phase depends on the temperature. Vapor pressures given in Table 1 will allow you to account for the contribution of $\ce{H2O}$ to the pressure.

Subtracting $n_i$ from $n_f$ leaves us with the molecules of $\ce{CO2}$. Therefore, the amount of $\ce{CO2}$ is obtained from:

When the reaction is done with standards CaCO₃

The stoichiometry of reaction (\ref{eq:reaction}) being 1 $\ce{CaCO3}$ : 1 $\ce{CO2}$, we have:

If we replace $n_{\ce{CO2}}$ and rearrange equation (\ref{eq:standard}) we can obtain the volume of the flask with the data from Table 2:

When the reaction with the antacid tablet is complete

Knowing the mole ratio of the reaction, and using the equations above, determine the mass of calcium carbonate in the sample of antacid that you tested.

For your results section you will need to determine the concentration of your unknown antacid tablet. You will need to calculate the concentration as percent by weight or $\%w/w$. To do this you take the mass of calcium carbonate that you determined above and divided by the mass of the piece of the antacid tablet.

Finally, in order to find the total mass of calcium carbonate in your antacid tablet, you will take the percent by weight and multiply it by the mass of the whole tablet.

**Table 1** - Vapour pressure of water
Temperature	Vapour Pressure	Temperature	Vapour Pressure
℃	kPa	℃	kPa
16.0	1.81	25.0	3.16
16.5	1.87	25.5	3.27
17.0	1.93	26.0	3.36
17.5	2.00	26.5	3.47
18.0	2.07	27.0	3.56
18.5	2.13	27.5	3.67
19.0	2.20	28.0	3.77
19.5	2.27	28.5	3.89
20.0	2.33	29.0	4.03
20.5	2.41	29.5	4.12
21.0	2.49	30.0	4.22
21.5	2.56	30.5	4.31
22.0	2.64	31.0	4.44
22.5	2.72	31.5	4.54
23.0	2.81	32.0	4.65
23.5	2.89	32.5	4.75
24.0	2.99	33.0	4.87
24.5	3.08	33.5	4.98

Gas Laws Datasheet

Hand one copy in before leaving lab and keep one copy for yourself

Personal Information First Name:

Last Name

Student ID number:

Demonstration:

Lab Section:

Date:

Device ID:

Table 2 - Reaction of HCl with standard CaCO_{3

Table 2 - Reaction of HCl with standard CaCO₃

Trial
Standard CaCO₃
Initial Values
Final Values

Pressure
Temperature
Pressure
Temperature

(#)
(g)
(kPa)
(℃)
(kPa)
(℃)

1.

2.

3.}

Standard CaCO₃ concentration:
Code # of antacid tablet:
Weight of the entire antacid tablet:

Table 3 - Reaction of HCl with antacid
Trial	Antacid	Initial Values		Final Values
		Pressure	Temperature	Pressure	Temperature
(#)	(g)	(kPa)	(℃)	(kPa)	(℃)
1.
2.
3.

Pre-Lab Questions (Compiled from the Lab Manual)

One of the non-active ingredients of TUMS brand antacid is sodium hexametaphosphate.Research two other uses of sodium hexametaphosphate.
Why do we use standard calcium carbonate to determine the volume of the flask? Why can't we use the the stated volume of the flask (125 mL) in our calculations?
Write out the procedure in your own words in point form. You do not need to include procedural elements related to the use of the Vernier interface. You may write it out by hand or type it and print it.

5%	Technique and prep	assessed by your TA during the lab
10%	Pre-Lab assignment	due by the beginning of the lab period
20%	Datasheet	hand in at the end of the lab period
10%	Lab quiz	myCourses before deadline
25%	Lab results	myCourses within 48 hours
30%	Lab report	myCourses within 48 hours

$P_i$	intial pressure measured in the flask before the reaction	kPa
$P_{Air-i}$	contribution of Air to the initial pressure	kPa
$P_{\ce{H2O}-i}$	contribution of $\ce{H2O}$ to the initial pressure, from Table 1	kPa
$P_f$	final pressure measured in the flask after the reaction	kPa
$P_{Air-f}$	contribution of Air to the final pressure	kPa
$P_{\ce{H2O}-f}$	contribution of $\ce{H2O}$ to the final pressure, from Table 1	kPa
$P_{\ce{CO2}-f}$	contribution of $\ce{CO2}$ to the final pressure	kPa

$P_i\phantom{l}^\prime$	initial pressure in the flask without $\ce{H2O}$ contribution	(kPa)
$P_f\phantom{l}^\prime$	final pressure in the flask without $\ce{H2O}$ contribution	(kPa)

$n_i$	molecules of gas initially in the flask. (Air contribution only)	(mol)
$n_f$	molecules of gas in the flask after reaction. (Air and $\ce{CO2}$ contribution only)	(mol)
$T_i$	initial temperature of the gas in the flask	(K)
$T_f$	final temperature of the gas in the flask	(K)
$V$	volume occupied by the gas, flask, tubing etc.	(L)
$R$	gas constant	(8.314 kPa L K^-1 mol^-1)

$m_{\ce{CaCO3}}$	mass of $\ce{CaCO3}$ in the flask before reaction	(g)
$\%\ce{CaCO3}$	standard concentration of $\ce{CaCO3}$
$MW_{\ce{CaCO3}}$	molecular weight of $\ce{CaCO3}$	(100.0869 g/mol)
$n_{\ce{CaCO3}}$	molecules of $\ce{CaCO3}$ initially in the flask	(mol)

Gas Laws

Learning Objectives

Suggested Reading

Introduction

Apparatus and Chemicals

Apparatus

Chemicals