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Tips for Solving Gas Law Problems
Dr. MJ Patterson
 When do you use the combined gas law, and when do you uses the ideal gas law?
The simplest answer is to look at how many sets of conditions you are given. For one set, use the ideal gas law. For two sets, use the combined gas law.
 Make a variable list.
When you first start working on a problem, write out all of the variables and the values assigned to them. This approach will make it clear as to which variable you are solving for. It will also become apparent if you are trying to use the wrong gas law.
 Convert all temperatures to Kelvin.
When working gas law problems, the temperature must always be in Kelvin.
 Convert all other units to the standard units.
To be safe, use the standard units  atms, liters, moles and Kelvin. With the combined gas law, you can sometimes get away with pressures and volumes in other units. With the ideal gas law, you must use the standard units for everything. So, to be safe, use the standard units. And remember, temperatures must always be in Kelvin  even in the combined gas law.
 What if the problem only says that the temperature stays constant, and doesn''t give a numerical value to plug in for the temperature?
For combined gas law problems, if the temperature stays constant, it cancels out of the equation. In other words, let''s say that T_{1} and T_{2} have the same value, and let''s call that number 100 K. When you plug 100 K into the combined gas law equation on both sides, it simply cancels out.
P_{1}V_{1}  =  P_{2}V_{2} 
n_{1}T_{1}  n_{2}T_{2} 

P_{1}V_{1}  =  P_{2}V_{2} 
n_{1}(100 K)  n_{2}(100 K) 

P_{1}V_{1}  =  P_{2}V_{2} 
n_{1}  n_{2} 

This same cancellation would occur regardless of what the value for the temperature actually is. The net result is that if the temperature is constant, it cancels out of the equation.
The same argument can be made for any of the other variables (P, V and n) as well. If it is constant throughout the process, it simply cancels from the equation.