SURVEY OF CHEMISTRY LABORATORY I CHEM 1151L DETERMINATION OF ATMOSPHERIC PRESSURE

CAUTION!!! BE SURE TO WEAR SAFETY GLASSES AT ALL TIMES IN THE LABORATORY NO EXCEPTIONS TO THIS RULE!

- To form and collect a sample of hydrogen (H 2 ) 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 H 2 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 H 2 gas only - Water vapor pressure can be determined using room temperature and Table 1 Atmospheric Pressure = calculated H 2 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 THE IDEAL GAS LAW

- Mass a piece of magnesium (Mg) metal - Make sure the mass is between g and 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 - H 2 gas will be evolved and trapped in the tube - Equal volume of water will be displaced by the H 2 gas - Make sure no gas escapes from the tube H 2 GAS EVOLES Mg + 2HCl → MgCl 2 + H 2 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 H 2 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 H 2 GAS

- Measure and record the room temperature (in o C) 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