Rodney Hahn

May 6th

Lab ?

Determining the Gas Constant “R”


Purpose: The basis of this experiment is the following reaction in which you will react a known mass of Magnesium with excess hydrochloric acid to produce the substances shown:


Mg + 2HCl → MgCl2 + H2


The hydrogen gas is the product that is of interest to you in this experiment. You will make an experimental determination of the number of moles of hydrogen molecules produced and the volume occupied by these molecules. The number of moles of hydrogen will be determined indirectly. The balanced equation for this reaction shows that the molar ratio of magnesium reacted to hydrogen gas produced is 1:1. Therefore, by determining the mass of magnesium that reacts and the number of moles that this mass is equal to, you will also determine the number of moles of hydrogen gas produced. The volume of hydrogen gas produced will be measured directly on the scale of a gas-measuring tube. The gas laws of Boyle and Charles will be used to correct this volume, measured under laboratory conditions, to the volume the sample of gas would occupy at STP. The collected data (number of moles and volume at STP) will be used to calculate the molar volume of the hydrogen gas. This experiment should aid in the understanding of the mole concept and the concept of molar volume of a gas.

Purpose: Determine the volume of 1 mole of hydrogen gas at STP using experimental data, known mathematical relationships, and a balanced chemical equation.

Equipment:gas-measuring tube and stopper, beaker 400-mL, ring stand, graduated cylinder, utility clamp, metric ruler, thermometer, and safety glasses.

Chemicals: magnesium ribbon (Mg); 3 M hydrochloric acid (HCl) Caustic!



  1. Obtain 2.3 – 2.6cm a piece of magnesium ribbon from your teacher. Measure the length in

millimeters. Record the length in your data table.


  1. Obtain a 50.00mL gas collection tube. Remove the rubber stopper with copper metal loop from the tube. Wrap the piece of magnesium around the copper loop so that it will fit easily into the gas-measuring tube.

  1. Add about 300 mL of water to a 400-mL beaker. Set up a ring stand and utility clamp, and place the beaker of water in the position shown in Fig. 29-2

  1. Obtain 10 mL of 3 M hydrochloric acid (HCI). (Caution): Handle this acid with care.) Carefully pour the HCI into a gas-measuring tube. Don’t spill. 5. Tilt the gas-measuring tube slightly. Using

  2. beaker, Slowlyfill the gas-measuring tube with water. Try to avoid mixing the acid and water as much as possible. 6. Insert the one-hole rubber stopper firmly into the tube as shown in Figure 29-1. 7. Place your finger over the hole in the rubber stopper and invert the gas-measuring tube. Lower the stoppered end of the tube into the beaker of water. Clamp the tube in place so that the stoppered end rests on the bottom of the beaker (Figure 29-2).

  1. Let the apparatus stand about five minutes after the magnesium has completely reacted. Carefully measure the volume of gas produced to the tenth of a milliliters.

  1. Place a thermometer in the 400ml beaker for 5 minutes and measure the water temperature.



Observations & Data table

(a) length of Mg ribbon________ mm (b) mass of 1000 mm of Mg =

(c)volume of H2 gas in tube ______ ml (d) water temperature ________ oC

(e) barometric pressure _____________ (from your instructor)

(f). water vapor pressure at room temperature __________ (see table in book)





Mass of 1000.0 mm Mg (g)


Pressure (mm HG)


Length of Mg (mm)


Temperature (oC)


Volume of Gas (ml)



Calculation Section


Finding mass of Mg reacted


23.0mm Mg (1.414/1000.0mm) =.0325 mol H2


Finding moles of H2 formed

Mg(S)+2HCl(aq) → MgCl2(aq)+H2(g)

.0325g Mg (1mol Mg/24.305)(1 mol H2/1 mol Mg)=.00134




PH2O at 20.0oC = 17.5 mm Hg

PH2O at 21.0oC = 18.7 mm Hg


Assume that the increase from 20.0oC – 21.0oC is constant

For every increase of temperature by 1.0oC, the pressure increases by 1.2mm Hg.

Therefore each .1oC increase in temperature, the pressure increases by .12 mm Hg


PH2O at 20.6oC

= 17.5 mm Hg + .72 mmHg




Finding partial pressure of H2



= P total – PH2O

=751.6mm Hg- 18.2mm Hg

=733.4mm Hg (1.000 atm/ 760.0mm Hg)

=.965 atm


Temperature of H2



= 20.6oC+ 273.2

=293.8 K


Volume of H2


= 35.91 ml (1L/103mL)

= .03594L


Finding Gas Constant “R”


=(.965 atm)(.03594L)/(293.8 K)(.00134)

=.0880 (atm x L)/(mol x K)


Finding percent error

% Error

= |Accepted Value Experimental value| x 100

Accepted Value

= .08206 – .0880 x 100%



=7.24% error





Mass of Mg (g)


Moles of H2 (mol)


Partial pressure H2O(g) (mm Hg)


Partial pressure of H2(g)


Temperature of H2(g)(k)


Volume of H2(g) (L)


Gas Constant “R”


Error %





Discussion of results

The results of this lab should have been finding the gas constant “R” which is .08206(atm x L)/(mol x K). The results of this lab concluded in being bigger than expected. The Gas constant we had a percent error of 7.24%. resulting in .0880(atm x L)/(mol x K) while it should have been .08206(atm x L)/(mol x K). This happened because as air bubbles leaked into the gas collecting tube in the graduated measuring cylinder causes the volume to be higher than its suppose to be. To fix this we should have shook the container to allow the air bubbles to come to the top. The difficulty of measuring the final volume (meniscus)- was the meniscus in the gas collecting tube could change do to where it was located if it where not water level. Our meniscus was located above water level in the 100 mL graduated measuring cylinder. If the angled end was included in the measurement of the length of the Mg then it would result in the volume of our gas being to small. Then if the angled in was not included in the measurement of the length of Mg then the volume would be to big because you would have an extra piece of magnesium making gas that your where not calculating in. In this the only solution is carefully cutting the magnesium carefully to 90o and make sure there are no bend s within the magnesium. The following Trapped air bubbles cause volume to be to low balancing out our air that escaped the gas collecting tube. If Mg was placed too close to the one-hole stopper, some gas escapes with the water as it leaves the gas collecting tube.



the purpose of this lab was to find “R”. we took magnesium reacted it with hydrochloride acid and collected the gas. The results of this lab should have been finding the gas constant “R” which is .08206(atm x L)/(mol x K). The results of this lab concluded in being bigger than expected. The Gas constant we had a percent error of 7.24%. resulting in .0880(atm x L)/(mol x K) while it should have been .08206(atm x L)/(mol x K). the major source of error in this lab was measuring the meniscus when it was above water level. Through this experiment my results in this experiment where not supported by the results we found.


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