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 Vmay 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= nRTor STP.
The values of P, T, and Vdiscussed 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 sub-
stance 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 con-
verted to moles.
The values of P, T, and nmay 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 Pand Talong
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 nwith the measured mass of a sample will allow you to calcu-
late 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
1.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.
2.In any of the gas laws, be sure to express the temperature in kelvin. Failure to do so is
a quite common mistake.
3.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 label-
ing your quantities (moles of O 2 instead of just moles).
4.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.
5.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.
6.Make sure your units cancel.
7.In using the combined gas equation, make sure you group all initial-condition quanti-
ties on one side of the equals sign and all final-condition quantities on the other side.
8.Be sure to use the correct molecular mass for those gases that exist as diatomic
molecules—H 2 , N 2 , O 2 , F 2 , Cl 2 , and Br 2 and I 2 vapors.
9.If the value 22.4 L/mol is to be used, make absolutely sure that it is applied to a gasat STP.Gases 113TIP