Gas Laws Lab REFERENCES: Brown & Holme, Sections 5.2, 5.3, 5.4, 5.5, 5.6 Gas laws describe the relationships between key variables characterizing gases, such as pressure (P), volume (V), temperature (T), and moles (n). Some gas laws relate only two variables at a time, others more: Boyles Law: P1V1 = P2V2
(constant n,T)
Avogadro’s Law: V1 = V2 (constant P,T) n1 n2 Ideal Gas Law: PV = nRT
Charles’ Law: V1 = V2 T1
(constant n,P)
T2
Combined Gas Law: P1V1 = P2V2 T1
(constant n)
T2
(R= universal gas constant = 0.0821 L-atm/mol-K = 62,400 mL-torr/mol-K)
The most useful of these is the Ideal Gas Law. Given three out of the four variables, the last can be calculated. All the other laws can be derived from the Ideal Gas Law. The purpose of this lab is two-fold: first, to recover the gas formed in a reaction, measuring its properties to determine its amount, and second, to use stoichoimetry to calculate the original amount of reactant. The reaction for this experiment is Mg (s) + 2 HCl (aq) H2 (g) + MgCl2 (aq) PROCEDURE: • Collect the following equipment: 1000 mL beaker, 50 mL buret, buret clamp and ring stand, thermometer, 10 mL graduated cylinder. • Select and unknown strip of Mg and record its letter. • Close the valve on the buret and add water up to the 50.00 mL mark. • Drain this into the graduated cylinder and record this volume. • Add 8.0 mL of 6.0 M HCl to the buret (make sure to close valve). • Continue filling the buret with distilled water all the way up to top. • Bend your Mg strip into a “V” and wedge it into the top of the buret. • Add more water if necessary to make the buret completely full. • Fill the 1000 mL beaker with about 750 mL water.
• With a finger on the top of the buret, flip the buret over and submerge it in the 1000 mL beaker. • Make sure the buret is all the way on the bottom of the beaker and secure in a clamp. • After the reaction is complete, record the difference in water levels between the buret and the beaker (in mm). • Record the buret reading at the water level in the buret. • Record the temperature of the water. • Look up the vapor pressure of water at your temperature in the CRC Handbook. • Record the atmospheric pressure. CALCULATIONS • Subtract the buret reading from 50 mL and add “the volume below the 50 mL mark” to get the total volume of H2 collected. • Express the pressure correction in mm Hg by dividing by the ratio of water density to mercury density: 1.00 g/mL 13.6 g/mL • Subtract the correction from the atmospheric pressure to get the corrected “wet” H2 pressure (total pressure in the buret) • Subtract the water vapor pressure to get the “dry” H2 pressure (pressure of just H2 in buret). • Calculate moles H2 formed using the Ideal Gas Law. • Calculate moles Mg reacted (use stoichoimetry). • Calculate mass Mg reacted (use molar mass).
GAS LAWS LAB REPORT FORM:
Name_________________________ Date_______________________
Show all calculations and units
data Unknown ID
trial one _______________
trial two _______________
Volume below 50 mL mark
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Difference in water levels
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Buret reading
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Temperature of water
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Vapor Pressure of Water at Temperature from Handbook
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Atmospheric pressure
_______________
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calculations Total volume of hydrogen collected
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Correction factor for pressure
_______________
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Corrected “wet” hydrogen pressure
_______________
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“Dry” hydrogen pressure
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Moles hydrogen formed
_______________
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Moles magnesium reacted
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Grams magnesium reacted
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Questions: 1. A sample of oxygen gas is produced in a reaction and collected under water in a graduated cylinder. The temperature is 28.3 oC and the volume of gas is 47.7 mL. During this experiment the atmospheric pressure is 0.98 atm. How many grams of oxygen were collected?
2. If 24.1 mL of N2 at 34.3 oC and 554 torr is allowed to expand to 89.6 mL and 776 torr, what is the new temperature?
3. In our experiment, if you did not correct the atmospheric pressure for the difference in water levels, how would that affect your calculated mass of magnesium? 4. In our experiment, if you did not correct the pressure in the buret for water vapor pressure, how would that affect your calculated mass of magnesium?
