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

Click on picture to play video

- 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

Click on picture to play video

- 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