LAB: Molar Volume of a Gas
Name_______________________________
Date_________ Block____
The basis of this lab is the reaction in which a known mass of magnesium metal reacts
completely with excess hydrochloric acid to produce aqueous magnesium chloride and hydrogen
gas according to the following balanced equation:
Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
Purpose: To determine experimentally the volume of 1 mole of hydrogen gas at STP.
Materials and Equipment:
gas-measuring tube
one-hole stopper
ring stand
utility clamp
Celsius thermometer
400-mL beaker
cotton thread
DI H2O(l)
50-mL beaker
paper towels
large deep container for equalizing gas pressure in tube
10-mL graduated cylinder
metric ruler
access to balance
safety goggles and apron
magnesium ribbon, Mg(s)
3.0 M hydrochloric acid, HCl(aq) (NOTE: 3.0 M means
3.0 moles of HCl(g) dissolved in 1.00 liter of solution)
wash bottle containing NaHCO3(aq)
SAFETY: Handle acids with care; ALWAYS wear safety goggles and lab
aprons when working with acids. Handle the gas-measuring tube with care to
avoid breakage.
To clean an acid spill: dilute the acid with sodium hydrogen carbonate
solution, NaHCO3(aq). Then wipe up using absorbent paper towel.
It is important to “work clean;” keep your lab surface clean and dry at all
times.
Read and follow the directions carefully and
completely!
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Procedure:
1. In the data table, record the mass of 1.00 m of magnesium ribbon as given by your teacher
2. Measure your piece of magnesium ribbon to the nearest 0.1 cm. Pay attention to sig. figs.
Record the length of the Mg(s) ribbon in the data table.
3. Use a piece of cotton thread about 15.0 cm long. Tie one end of the thread around the Mg(s).
Bend the Mg(s) so it will fit easily inside the gas-measuring tube.
4. Measure about 10.0 mL HCl(aq). Caution: handle acids with care!
HCl(aq) into the gas-measuring tube.
Carefully pour the
5. Pour about 40. mL DI H2O(l) into the 50-mL beaker. Tilt the gas-measuring tube.
SLOWLY pour the DI H2O(l) into the gas-measuring tube. Try to avoid mixing the acid and
water as much as possible.
6. Carefully hold the gas-measuring tube upright and add enough DI H2O(l) to completely fill
the gas-measuring tube. Gently lower the Mg(s) about 4-5 cm into the gas-measuring tube
and drape the thread over the side of the tube. Carefully insert the one-hole stopper into the
mouth of the gas-measuring tube; you do not want any trapped air bubbles. Set the gas
measuring tube in the sink so it remains at a slant but upright.
7. Add about 300-mL water into the 400-mL beaker. Set up the ring stand and utility clamp, and
then place the 400-mL beaker in position as shown in the set-up on the demo table.
8. Place your index finger over the hole in the rubber stopper and invert the gas-measuring tube.
Lower the stoppered end of the tube into the 400-mL beaker of water. Once the stopper is
under water you may remove your finger from covering the hole. Clamp the tube in place so
that the stoppered end is a FEW cm (about 2-3 cm) above the bottom of the beaker.
9. Allow the reaction to go to completion (no more gas bubbles are being formed) and then wait
another 2-3 minutes. Record your visual observations in the data table.
10. Unclamp the gas-measuring tube keeping the stoppered end under water. Place your index
finger over the hole in the stopper under water. Carefully lift the tube out of the beaker.
11. Keeping your finger over the hole in the stopper, VERY SLOWLY tilt the tube so that the
solution in the tube “runs” to the other end of the tube. Then, still with your finger over the
stopper hole, again VERY SLOWLY tilt the tube so that the solution “runs” to the
stoppered end of the tube. This is the BEST method for “catching” the many gas bubbles
that formed on the inside of the gas-measuring tube during the reaction. If there are still
some gas bubbles on the inside of the tube, repeat the tilting action.
12. Carefully lower the stoppered end of the gas-measuring tube into the large cylinder of water
in the sink at the demo table or in the container in the sink at the back of the room. Once
the stoppered end of the tube in under water, you may remove your finger from the stopper
hole.
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13. Move the gas-measuring tube up or down in the water until the water level INSIDE the gasmeasuring tube is the same as the water level outside of the tube. This equalizes the
pressure of the gas inside the tube with the atmospheric pressure outside of the tube.
14. CAREFULLY measure the volume of the GAS inside the gas-measuring tube to the nearest
0.5 mL. (Remember- you want to read the measurement at the meniscus of the water in the
tube.) Record your measurement (with sig. figs.!) in the data table.
15. In the data table, record the room temperature, water vapor pressure at the given roomt
temperature, and the atmospheric pressure as given by your teacher.
16. Repeat Steps 2-15 for a total of 4 trials.
Data Table (30 points – 1 point each value for correct set of sig. figs.)
Measurement Taken
Trial #1
Trial #2
Trial #3
Trial #4
Average
length of Mg(s) ribbon used –
measured to nearest 0.1 cm
mass in g of 1.00 m of Mg(s)
ribbon (as written on board)
measured volume of H2(g)
produced (mL)
room temperature (oC)
(as written on board)
atmospheric pressure
(measured in mmHg as written
on board)
water vapor pressure at
measured room temperature
(in mmHg as written on board)
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Calculations: Use the AVERAGE VALUES from the Data Table. You MUST show all
work. A “dummy” set-up for any calculation that we have not done before is given where
needed. Follow the set-up! Pay attention to sig. figs.!
(Points for each calculation are given.)
values for each measurement.
NOTE: the ~ symbol represents YOUR numerical
1. Calculate the mass of your strip of Mg(s): ~cm Mg x
1 m__ x ~ g Mg(s) = ~ g Mg(s)
100 cm
1m
(6 points)
2. Calculate the number of moles of Mg(s) that reacted. (This is a mass-mole conversion.)
NOTE: According to the balanced chemical equation, the number of moles Mg(s) that
react is equal to the number of moles H2(g) produced.
(4 points)
4
3. Calculate the pressure of the H2(g) in the tube. P is the pressure value in mmHg.
PH (g) = Patmospheric ─ Pwater vapor
2
(4 points)
PH (g) =
2
4. Convert room temperature from oC to Kelvin. (sig. figs.!!!!)
(4 points)
K=
5. Calculate the volume of the H2(g) produced at STP (standard temperature and
pressure):
V2 = ( V1 x P1 x T2 )
( T1 x P2 )
where P1 is the pressure of the H2(g) in the gas-measuring tube (from calculation #3)
V1 is the measured volume of H2(g) (from the Data Table)
T1 is the room temperature in Kelvin (from calculation #4)
P2 is standard pressure: 760.0 mmHg
T2 is standard temperature: 0.0 K
V2 is the calculated volume of the amount of H2(g) produced at STP
Be sure you correctly identify your values! Enter your values in the spaces provided – with
units! (3 points)
P1 = _____________
P2 = 760.0 mmHg
V1 = _____________
V2 = ? mL
T1 = _____________
T2 = 0.0 K
The calculation is to be done in the box provided on page 6.
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Calculation #5 is done in this box (12 points)
6. Calculate the volume (in mL) of 1 mole H2(g) at STP using your answers from the
above calculations.
Volume in mL of 1 mole H2(g) at STP =
(5 points)
6
(1 mole x ~ mL H2(g) produced at STP)
~ of moles H2(g) produced
7. Using the volume in mL of H2(g) produced at STP, convert mL H2(g) to L H2(g).
1 L = 1 000 mL
L H2(g) = # mL H2(g) x
1 L___
1 000 mL
(4 points)
8. Calculate your percent error.
Remember UNITS!
Percent values are expressed to 2 decimal places.
The accepted value for the volume of 1 mole of any gas is 22.4 L.
% error = ( experimental value ─ accepted value ) x 100
accepted value
(5 points)
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Conclusions and Analysis
1. What are some possible sources of error in this lab? (Sources of error are those things
over which you have no control. This means you can not include errors in measuring or
errors in following the directions!) (5 points)
2a) What is the other product of the chemical reaction in this lab? (2 points)
b) What happened to the other product of the reaction in this lab? (3 points)
(a)
(b)
3. This lab could not be done if the gas produced was oxygen because oxygen is quite
soluble in water. What conclusion can be drawn about the solubility of hydrogen?
(5 points)
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4. Give one reason why the hydrogen was collected over water. (5 points)
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