Collecting Gases Over Water
Lab #____
Name___________________________
Lab Partners_____________________
Purpose: To determine mass of magnesium ribbon and standard dry volume of hydrogen gas.
Materials:
Eudiometer
400-mL beaker
3 M HCl
Barometer
Ring stand
Rubber stopper
Mg Ribbon
Metric Ruler
Clamp
Copper wire
Thermometer
Procedure:
1. Fill a 400-mL beaker about 3/4 full with room temperature water.
2. Cut a piece of Mg ribbon that is about 5 cm long. Carefully measure the length of the ribbon to
+/- 0.1 cm and record your data.
3. Bend the Mg ribbon so that it can fit in the end of the Eudiometer and wind it with copper wire.
4. Prepare the ring stand and clamp for the Eudiometer.
5. Slowly pour 10 mL of 3 M HCl into the Eudiometer. Pour the room temperature water from
your 400-mL beaker into the Eudiometer and fill to the top of the tube. Take care to
minimize mixing with the HCl at the bottom.
6. When the Eudiometer is completely full with water, place the coiled Mg ribbon about 3 to 4 cm
into the Eudiometer. With the copper wire against the side of the tube, insert the rubber
stopper. Take care not to introduce any air bubbles into the tube!
7. With your finger over the hole in the rubber stopper, invert the Eudiometer and place into the
400-mL beaker of (3/4 full) water.
8. Clamp the apparatus to the ring stand, with the bottom of the rubber stopper about 1 – 2 cm
from the bottom of the beaker.
9. Allow all of the Mg ribbon to react. When no more bubbles are seen, tap the sides of the
Eudiometer to make sure all of the gas has been collected.
10. Determine the volume of gas collected to the correct number of sig figs. Record your data.
11. Use your metric ruler to measure the height difference between the liquids in the tube and
beaker. Record your data.
12. Record the room temperature and atmospheric pressure.
13. Pour the liquids into the sink and rinse everything with tap water. Clean your station and
place all materials on the cart in the back of the room.
Data:
Length of Mg ribbon:
_________
Mass of 1.00 meter of Mg ribbon:
_________
Volume of wet gas:
_________
Height difference of liquids:
_________
Room temperature:
_________
Atmospheric pressure:
_________
Water vapor pressure:
_________
Calculations:
1. Write the balanced reaction for the lab. Include states of matter.
2. Use the g/cm ratio provided by your teacher to determine the mass of the magnesium ribbon.
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3. Calculate the moles of magnesium reacted.
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4. Calculate the dry volume of the hydrogen gas under laboratory conditions.
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5. Calculate the dry standard volume of the hydrogen gas.
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6. Using the moles of hydrogen gas produced and the dry standard volume calculated in #5,
determine the experimental volume of 1 mol of gas at STP.
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7. Use the accepted value of the molar volume of a gas at STP of 22.414 L to calculate your %
error.
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8. A student reacts an unknown amount of magnesium ribbon with excess HCl. He collects 40.7
mL of hydrogen gas over water at 24.0°C and 101.5 kPa. If the water in the gas collecting tube is
13.6 cm higher than the surrounding water, how many grams of Mg did he react?
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