Here in this example question, it says, how many moles of ammonia are contained in a 25-liter tank at 190 degrees Celsius and 5.20 atmospheres. Alright. So they're asking me to find moles of ammonia. So that would be my n variable. They're giving me liters, so that would be volume. They're giving me degrees Celsius which is temperature, and they're giving me atmospheres which is a unit for pressure. We can see that we have PV = nRT, so we have the ideal gas law to deal with. And the variable that's missing, the one that we're looking for is moles. So all we have to do here is isolate moles. So divide both sides here by RT, and then we're going to say here that moles equals PV over RT. Pressure is already in atmospheres, so no need to convert. Then we're going to have here volume which is already in liters so no need to convert. Then we have our R constant, remember, is 0.08206. And now that we know that we're using the ideal gas law, we have to use the units of liters times atmospheres per moles times Kelvin.

Temperature, remember, has to be in Kelvin. So there is a little bit of converting here necessary. So we have 190 degrees Celsius, so add 273.15 to that and that'll give us 463.15 Kelvin. So take those and plug it in, 463.15 Kelvin. So what cancels out? Atmospheres cancel out, liters cancel out, Kelvins cancel out, and you'll see that the only unit left are the moles that we need to find. So we're going to get initially 3.4205 moles. If we look back on the question, this has 3 significant figures, 2 significant figures, and 3 significant figures. We go with the least number of significant figures, so we're going to put our answer as 3.4 moles of ammonia. So this would be our final answer. So just realize what variables you are being given, what's the missing variable, just use your algebra skills to isolate that one variable, and you'll get your answer at the end.