Gases: Common Mistakes to Avoid for AP Chemistry (page 2)
Gas law experiments generally involve pressure, volume, and temperature measurements. In a few cases, other measurements such as mass and time are necessary. You should remember that ΔP, for example, is NOT a measurement; the initial and final pressure measurements are the actual measurements made in the laboratory. Another common error is the application of gas law type information and calculations for non-gaseous materials. Typical experiments involving these concepts are experiments 3 and 5.
A common consideration is the presence of water vapor, H2O(g). Water generates a vapor pressure, which varies with the temperature. Dalton's law is used in these cases to adjust the pressure of a gas sample for the presence of water vapor. The total pressure (normally atmospheric pressure) is the pressure of the gas or gases being collected and the water vapor. When the pressure of an individual gas is needed, the vapor pressure of water is subtracted from the total pressure. Finding the vapor pressure of water requires measuring the temperature and using a table showing vapor pressure of water versus temperature.
In experiments on Graham's law, time is measured. The amount of time required for a sample to effuse is the measurement. The amount of material effusing divided by the time elapsed is the rate of effusion.
Most gas law experiments use either the combined gas law or the ideal gas equation. Moles of gas are a major factor in many of these experiments. The combined gas law can generate the moles of a gas by adjusting the volume to STP and using Avogadro's relationship of 22.4 L/mol at STP. The ideal gas equation gives moles from the relationship n = PV/RT.
Two common gas law experiments are "Determination of Molar Mass by Vapor Density" and "Determination of the Molar Volume of a Gas." While it is possible to use the combined gas law (through 22.4 L/mol at STP) for either of these, the ideal gas equation is easier to use. The values for P, V, T, and n must be determined.
The temperature may be determined easily using a thermometer. The temperature measurement is normally in °C. The °C must then be converted to a Kelvin temperature (K = °C + 273).
Pressure is measured using a barometer. If water vapor is present, a correction is needed in the pressure to compensate for its presence. The vapor pressure of water is found in a table of vapor pressure versus temperature. Subtract the value found in this table from the measured pressure (Dalton's law). Values from tables are not considered to be measurements for an experiment. If you are going to use 0.0821 L atm/mol K for R, convert the pressure to atmospheres.
The value of V may be measured or calculated. A simple measurement of the volume of a container may be made, or a measurement of the volume of displaced water may be required. Calculating the volume requires knowing the number of moles of gas present. No matter how you get the volume, don't forget to convert it to liters when using PV = nRT or STP.
The values of P, T, and V discussed above may be used, through the use of the ideal gas equation, to determine the number of moles present in a gaseous sample. Stoichiometry is the alternate method of determining the number of moles present. A quantity of a substance is converted to a gas. This conversion may be accomplished in a variety of ways. The most common stoichiometric methods are through volatilization or reaction. The volatilization method is the simplest. A weighed quantity (measure the mass) of a substance is converted to moles by using the molar mass (molecular weight). If a reaction is taking place, the quantity of one of the substances must be determined (normally with the mass and molar mass) and then, through the use of the mole-to-mole ratio, this value is converted to moles.
The values of P, T, and n may be used to determine the volume of a gas. If this volume is to be used with Avogadro's law of 22.4 L/mol, the combined gas law must be employed to adjust the volume to STP. This equation will use the measured values for P and T along with the calculated value of V. These values are combined with STP conditions (0°C (273.15 K) and 1.00 atm) to determine the molar volume of a gas.
Combining the value of n with the measured mass of a sample will allow you to calculate the molar mass of the gas.
Do not forget: Values found in tables and conversions from one unit to another are not experimental measurements.
Common Mistakes to Avoid
- When using any of the gas laws, be sure you are dealing with gases, not liquids or solids. We've lost track of how many times we've seen people apply gas laws in situations in which no gases were involved.
- In any of the gas laws, be sure to express the temperature in kelvin. Failure to do so is a quite common mistake.
- Be sure, especially in stoichiometry problems involving gases, that you are calculating the volume, pressure, etc. of the correct gas. You can avoid this mistake by clearly labeling your quantities (moles of O2 instead of just moles).
- Make sure your answer is reasonable. Analyze the problem; don't just write a number down from your calculator. Be sure to check your number of significant figures.
- If you have a gas at a certain set of volume/temperature/pressure conditions and the conditions change, you will probably use the combined gas equation. If moles of gas are involved, the ideal gas equation will probably be useful.
- Make sure your units cancel.
- In using the combined gas equation, make sure you group all initial-condition quantities on one side of the equals sign and all final-condition quantities on the other side.
- Be sure to use the correct molecular mass for those gases that exist as diatomic molecules—H2, N2, O2, F2, Cl2, and Br2 and I2 vapors.
- If the value 22.4 L/mol is to be used, make absolutely sure that it is applied to a gas at STP.
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