how do u know when to use the ideal gas laws instead of the combined gas laws?

Question

And also how do u solve for Molar mass using the ideal gas laws?

Answer 1

In the combined gas law, you will have three things changing: volume, pressure and temperature. Typically, you will be asked to calculate a new volume and the new T and P will be standard values.

With the ideal gas law, nothing changes. There will be only one pressure, temperature and so on. You will be asked to calcuate some unknown value (often it is moles) from the given set of data.

http://dbhs.wvusd.k12.ca.us/webdocs/GasLaw/Gas-Ideal.html

Look at example #1 and #2. Also, some of the worksheet problems are about calculating the molar mass using the ideal gas law. I can assure you that this type of problem will be on your upcoming test.

Answer 2

You do this then the pressure is low , meaning that the amount of gas is not very high. In standard condition the ideal formula is a good approximation.
You can determine M with the following. you measure the pressure, volume and temperature. You find n number of moles of gas. and knowing the weight of gas used , you divide the weight by n