SURVEY OF CHEMISTRY LABORATORY I
CHEM 1151L
DETERMINATION OF ATMOSPHERIC PRESSURE
CAUTION!!!CAUTION!!!BE SURE TO WEAR SAFETY GLASSES AT
ALLTIMES IN THE LABORATORY
NO EXCEPTIONS TO THIS RULE!
- To form and collect a sample of hydrogen (H2) gas through a chemical reaction
- Students will measure the volume of hydrogen gas
- Students will use the room temperature and the calculated moles of the H2 gas to determine the atmospheric pressure
DETERMINATION OF ATMOSPHERIC PRESSURE
- The pressure of the gas is the total pressure of all gases present
- Hydrogen gas is collected over water so contains some water vapor
- Ideal gas relationship will give pressure due to the H2 gas only
- Water vapor pressure can be determined using room temperature and Table 1
Atmospheric Pressure = calculated H2 pressure + water vapor pressure
DETERMINATION OF ATMOSPHERIC PRESSURE
PV = nRT
P = pressure of a gas (1 atmosphere = 760 mm Hg)
V = volume of the gas expressed in liters (L)
n = moles of the gaseous substance
T = temperature of the gas in Kelvin (K = ˚C + 273)
R = gas constant
Kmoleatmliter0.08205R
THE IDEAL GAS LAW
- Mass a piece of magnesium (Mg) metal
- Make sure the mass is between 0.030 g and 0.050 g
- Record the mass
MAGNESIUM METAL
- Bend the Mg metal strip
- Wrap some copper wool loosely around the Mg metal
- Tie the bundle securely with about 2-3 ft of string
MAGNESIUM METAL
- Obtain a ring stand and a buret clamp
- Also obtain a 1000 mL beaker
- Fill the beaker with water (about half-filled)
- Set them aside
OBTAIN BEAKER AND STAND
- Using the dispensing bottle
- Dispense about 10 mL of 12 M hydrochloric acid (HCl)
directly into a gas collection tube
FILL TUBE WITH HCl
- Carefully and quickly fill the rest of the tube with distilled
water
- Avoid mixing
FILL WITH WATER AND SUBMERGE SAMPLE
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- Submerge the metal sample in the tube to the 50 mL mark
- Completely fill the tube
FILL WITH WATER AND SUBMERGE SAMPLE
- Cover the tube opening with your finger and invert the tube
- Immediately insert the tube into the beaker of water
- Take finger off when the tube is completely immersed in the water
- Place the gas tube in the buret holder clamp and adjust the gas tube so it is just touching the bottom of the
beaker
IMMERSE TUBE IN BEAKER
- The HCl will move slowly down the tube
- HCl will react with Mg upon contact
- H2 gas will be evolved and trapped in the tube
- Equal volume of water will be displaced by the H2 gas
- Make sure no gas escapes from the tube
H2 GAS EVOLES
Mg + 2HCl → MgCl2 + H2
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- No more gas bubbles will be given off when reaction is
complete
- That is Mg is completely used up
- Tap the tube to dislodge any clinging gas bubbles
REACTION GOES TO COMPLETION
- Unclamp the gas tube and lift slightly (open end still under
water)
- Cover the open end with your finger and remove from the
beaker
- Gas collection tube should remain in the inverted position
MEASURE VOLUME OF H2 GAS
- Transfer tube to a large graduated cylinder filled with water
- Remove your finger once the end of the tube is under water
- Immerse the gas tube until the liquid level in the gas tube is even with the liquid
level in the large container
- Read and record the volume of the hydrogen gas (in mL) from the gas
collection tube
MEASURE VOLUME OF H2 GAS
- Measure and record the room temperature (in oC) around the time
volume measurement is taken
- Do not hold thermometer by the bulb
- Use the temperature to determine the water vapor pressure from Table 1
in the manual
MEASURE ROOM TEMPERATURE
- Remove and dispose of the copper wadding
- Complete all calculations
COMPLETE CALCULATIONS AND CLEAN UP