Lesson 7-6

Combined Gas Law Problems

     There are many situations that would call for the use of more than one of the gas laws.  Perhaps the most common situation involves using both Charles's law and Boyle's law on the same gas sample.   Because pressure and temperature will change from day to day and from location to location, it is common to mathematically adjust the volume of a gas to standard temperature and pressure.  In such a situation, the values for STP are used for T₂ and P₂.  The formula for the combined gas law is shown below:

            V₁P₁T₂
   V₂ =  -----------
              P₂T₁

     Example 1.  A 350 cm³ sample of helium gas is collected at 22.0 ^oC and 99.3 kPa.  What volume would this gas occupy at STP?

Solving:

First, you must change the Celsius temperature to Kelvin.

A 350 cm³ sample of helium gas is collected at 22.0 ^oC 295 K and 99.3 kPa.  What volume would this gas occupy at STP?

Now, list the givens and the unknown

V₁ = 350 cm³
P₁ = 99.3 kPa
T₁ = 295 K
V₂ = ?
P₂ = 101.3 kPa (standard pressure)
T₂ = 273 K (standard temperature)

Now, substitute the values into the equation, which is already set up for volume 2 as the unknown.

V₂ = V₁P₁T₂
           -----------
              P₂T₁

V₂ = 350 cm³ x 99.3 kPa x 273 K
               --------------------------------------
101.3 kPa x 295 K

V₂ = 317.5034718 cm³

V₂ = 320 cm³

Now, practice what you have learned by following the links below.

Please forward all questions, comments and criticisms to Gregory L. Curran.
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Last Modified February 07, 2008