AAMC Chem Q Pack #83

[deleted647690]

This is a basic concept that I'm getting confused about

When I first solved this, I found that .003 mol PH3 would be produced:

.002 mol R2NBCl2 * 3 mol PH3 / 2 mol R2NBCl2 tells you that .003 mol pH3 will be produced.

I thought I would need to plug this into the ideal gas law to find the final volume, thus:

V = nRT/P
.003 mol (.08) (273 K) / 1 atm ==.065 L ==65 L

However, to actually solve this, you need to know that 1 mol = 22.4 L at STP. Thus

.003 mol PH3 * 22.4 L/mol = About 67 mL, choice C



What am I confusing here? I think I'm confused about what types of questions you would use the ideal gas law for. Perhaps this was just supposed to be a stoichiometry question

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[aldol16]

You're confusing atmospheric pressure for gas pressure. PV = nRT refers to the pressure of the gas, that is, the pressure the gas is exerting. 1 atm is not the pressure the gas is exerting but rather the pressure that the atmosphere is exerting on the system. So you don't know P in this case, which means you can't use the ideal gas law.

 

[deleted647690]

You're confusing atmospheric pressure for gas pressure. PV = nRT refers to the pressure of the gas, that is, the pressure the gas is exerting. 1 atm is not the pressure the gas is exerting but rather the pressure that the atmosphere is exerting on the system. So you don't know P in this case, which means you can't use the ideal gas law.

Oh, thanks! That clears it up

 

[deleted647690]

You're confusing atmospheric pressure for gas pressure. PV = nRT refers to the pressure of the gas, that is, the pressure the gas is exerting. 1 atm is not the pressure the gas is exerting but rather the pressure that the atmosphere is exerting on the system. So you don't know P in this case, which means you can't use the ideal gas law.

I tend to make that error sometimes. I don't even understand what the question is asking, and I just see the variables and then jump to whatever formula pops into my head.

Oh, I see mols, temp, pressure, volume; must be an ideal gas equation question! That was probably my thought process on this question, especially considering I was trying to keep up a pace with these