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5 Tips on How to Solve Gas Law Problems - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Here are five small reminders for you to notice before your gas laws unit test. These are five things that my students tend to make mistakes on. I'm sure that this will help you as well.

The first one; temperature is always in Kelvin never in Celsius. It might be given in Celsius in your problem, but you have always have to convert it to Kelvin. Don't forget Kelvin is 273 + Celsius. If it is not in Kelvin you will be wrong.

Two; notice, I want you to notice, if you're given moles, or given Mass of a gas, you must use the Ideal Gas Law. If you're not given moles or mass of a particular gas, you are not to use Ideal Gas Law. Let's write that down. Notice the only gas law with moles or mass in it as a variable, is Ideal Gas Law. Remind ourselves that Ideal Gas Law is PV=nRT.

If you're not given moles or mass, or not asked to calculate Moles or Mass, do not use the Ideal Gas Law. If you are given Moles or Mass, or asked to calculate Moles or Mass, the only thing you can use is Ideal Gas Law. Make sure you're aware of that. It will help you differentiate when and where to use it.

I also want you to know that one Mole of gas is only 22.4L at STP. What is STP? STP is 1 atmosphere at 273Kelvin, that's the condition at STP. So if your gases are not in these condtions, it is not 22.4L it is only 22.4L if it'S at STP. If your problem does not state that the conditions are STP, you cannot assume that your gas takes a 22.4L of space. You must otherwise get to calculate it. You must calculate if it's not. You can calculate using one of the gas laws; Boyle's, Charles and so on.

The next one. Speaking of gas laws and Ideal Gas Laws, in there is this R. What the heck is that? It's the gas constant. You want to make sure you use the proper gas constant when you're with the proper unit of pressure. You're going to look at your unit of pressure and if it's in atmposheres, you're going to use .0821 as your gas law constant. And the unit for that is atm L/mol.K and this is just so you can cross it out.

If you're given units in Millimeters in Mercury (mmHg/torr exactly the same thing). Millimeters and mercury is a measurement of the mercury. Torr is the man who invented that method. The units are 62.4 mmHg.l/mol.K. Your teacher may be very peaky on making sure you know your units, make sure you have that.

The last one that you're probably going to be dealing with is kilopascal (kPa) and that is 8.314 kilopascal Litre per Mole Kelvin. If your units, get what units of pressure you're dealing with, and figure out what unit, what R you're actually going to use. You can always convert if you want to 1 over 1 and always use .0821 but you have to notice your units to be aware.

Last point. Speaking of units, make sure all of the units of pressure are the same in the problem. In other words, check your units. So if you're given, if for example if you're using Boyle's Law; PV=PV, you want to make sure units of pressure are the same. If you need to covert here are the conversions; 1atm=760mmHg. We're also saying that's torr, if your teacher is using that. And this all equals to 101.3 kPa, these are three common units of pressure used.

Make sure also your volumes are the same. Tour volumes are consistent. If you're using Millilitres, make sure it's Millilitres. If you're using Litres, make sure it's Litres. But I also want you to know that 1cm³=1mL they're the same thing, they're equal to each other. This will help you with your volumes.

These are the 5 things that I want you to notice when you're walking into your exam and asked to solve some problems. Those are the things that most students mess up on. So make sure you're very aware of 5 these things. Hope that helped.