Determining the Value of the Gas Constant, R
Introduction:
This experiment will use the reaction of magnesium metal and
hydrochloric acid (HCl) to produce hydrogen gas. An excess of HCl will be
used to insure that all the magnesium will react. From the number of
moles of Mg used, the number of moles of H2 produced can be
calculated. From using the number of moles of H2 and measuring the
pressure, temperature, and volume of the H2, the value for R can then be
calculated.
Procedure:
1.
Steel wool a 0.042 – 0.047 g (about 1-3 inches) strip of Mg ribbon
to remove any oxide coating. Weigh it to 0.001 grams and record,
being sure it is in the listed mass range. Tie a piece of string to it.
2.
Fill a large beaker about ½ full of H2O. Measure about 15 ml 6 M
HCl in a graduated cylinder. Pour it through a funnel into a gas
measuring tube and then fill it completely to the brim with H2O,
being careful not to mix the HCl with the H2O any more than is
necessary.
3.
Place the Mg to a depth of about two inches in the tube, leaving the
string hanging over the edge; refill the tube with H2O to the brim.
4.
Quickly cover the end of the tube with your finger, invert it, and
submerge it in the beaker of water so the tube just touches the
bottom of the beaker, holding the string in place (the Mg must
remain in the tube!). Clamp the tube into position.
5.
The HCl is heavier than the H2O and will start diffusing down
toward the Mg as soon as the tube is inverted. When it contacts
the Mg, the bubbles of H2 will be produced. When the reaction is
complete (as seen by the disappearance of the Mg), tap any air
bubbles off the side of the tube.
6.
Let the apparatus cool to room temperature (about 5 minutes).
2
7.
Raise or lower the tube until the water levels outside and inside are
the same and then record the volume of H2 in the tube. (If your
beaker is too small to do this, transfer the entire apparatus to a
tall graduated cylinder full of H2O and equalize the water levels.
(Get the instructor's help here, if needed.)
8.
Record the room temperature and pressure. (Have your instructor
show you where the barometer is or write today’s pressure for you
on the board.)
3
Name
Lab Day
Report for Gas Constant Experiment:
Data:
1.
weight of magnesium ribbon
2.
volume of H2 gas (pressure equalized)
3.
room temperature
4.
room pressure
5.
water temperature
g
mL
ºC
torr
ºC
Calculations:
1.
a.
Write the balanced equation for the reaction of Mg with HCl.
b.
From your observations, is the magnesium the limiting
reactant? How do you know?
4
2.
3.
In calculating the value of n, many students obtain the value of
PV
moles of H2 to use from the equation: n 
.
RT
a.
Why shouldn't you do this?
b.
How should you calculate the moles H2 in this experiment?
Do it. (Watch significant digits!)
There are two gases in the tube: H2 and H2O vapor.
a.
How did the water vapor get there?
b.
The table on the next page shows the partial pressures of
water vapor at various temperatures. Calculate the partial
pressure of:
c.
1)
water vapor in the tube.
2)
H2 in the tube.
In calculating the value of R, you will use the moles H2.
What pressure should you use: total pressure in the tube,
PH2O , or PH2 ?
5
Temperature ºC
Vapor pressure of H2O in torr
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
9.2
9.8
10.5
11.2
12.0
12.8
13.6
14.5
15.5
16.5
17.5
18.7
19.8
21.1
22.4
23.8
25.2
26.7
28.3
30.0
31.8
4.
Calculate the value of R to the appropriate number of significant
digits from your experimental data in units of liter-atm
mole-K .
5.
Calculate your percent error. (The accepted value for R is
0.08206 liter-atm
mole-K ).
% error =
theoretical  actual
theoretical
× 100 =
0.08206  step # 4
0.08206
× 100