1. Combined Gas Law Problems
Using

2. Basic Information
These types of problems use six different variables: V1 (volume), P1 (pressure), T1 (temperature), V2 (volume), P2 (pressure), and T2 (temperature). The subscripts indicate where the variable is in the problem. The variables with the subscript of 2 come after the variables with the subscript of 1.

3. Simply cross-multiply, divide, and get your answer of 800 mL.
Easy Example Problem
A gas has a volume of mL at °C and torr. What would the volume of the gas be at °C and torr of pressure?
First, convert degrees Celsius to Kelvin with the conversion factor K = °C P1 = torr, V1 = mL, T1 = K, P2 = torr, V2 = ?, and T2 = K.
Simply cross-multiply, divide, and get your answer of 800 mL.
And that’s basically all there is to combined gas law problems, well, other than harder ones.

4. Don’t forget to convert to Kelvin before working the problem out.
Medium Problem
A gas sample occupies 3.25 liters at 24.5 °C and 1825 mm Hg. Determine the temperature at which the gas will occupy 4250 mL at 1.50 atm.
This problem is the same as the previous one with the exception of the pressure units and volume units. You simply need to convert mL to L and atm to mm Hg before you do anything else. Using the information;
1 L = 1000 mL and 1 atm = 760 mm Hg, to convert the units, it is found that 1.50 atm = 1140 mm Hg and 4250 mL = 4.25 L.
Don’t forget to convert to Kelvin before working the problem out.

5. Hard Problem
690.0 mL of oxygen are collected over water at 26.0 °C and a total pressure of mm Hg. What is the volume of dry oxygen at 52.0 °C and mm Hg?
This one requires a little bit more thinking. Because the oxygen is collected over water, water vapor pressure and Dalton’s law come into effect. First thing to do is find the water vapor pressure at 26.0 °C here. It is 25.2; so subtract 25.2 from mm Hg (699.8 mm Hg) to get the pressure of just the oxygen. Now convert °C to Kelvin by adding 273 to the Celsius degrees to get 299 K and 325 K. So, P1 = mm Hg, V1 = mL, T1 = 299 K, P2 = mm Hg, V2 = ?, and T2 = 325 K.

6. Hard Problem Continued
Cross multiply and then divide to get mL.