Chem Qpack #85

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931287m
Posts: 93
Joined: Mon Mar 11, 2019 1:43 pm

Chem Qpack #85

Post by 931287m » Thu Jul 04, 2019 10:50 pm

Hello,

What is the easiest way to go about solving this question? Why is it that the compound with a higher molecular weight will contain fewer moles?

I read an explanation online about comparing the ideal gas law (V = (nR/P)T) to y = mx+b. A decrease in moles therefore leads to a decreased slope (nR/P). Is there another way to go about this one? I feel like I would have never thought to approach the question this way

Thanks in advance!
NS_Tutor_Mathias
Posts: 264
Joined: Sat Mar 30, 2019 8:39 pm

Re: Chem Qpack #85

Post by NS_Tutor_Mathias » Fri Jul 05, 2019 4:41 pm

> Why is it that the compound with a higher molecular weight will contain fewer moles?

The bluntest way to put this would be that #moles = (mass of compound)/(molar mass)

A slightly more intuitive way to say all that is that for 1 mole of the heavier compound weighs more than 1 mole of the lighter compound (duh), but they both contain the exact same number of molecules (and uh, moles: 1). Therefore the same MASS of both (1 g) would inevitably mean fewer molecules (and therefore moles) of the heavier compound.

For the explanation pointing out that the gas law for only 2 variables is a linear equation: Of course. But if you're not very comfortable with that, just take your ideal gas law (PV=nRT) and practice by graphing it for different givens: What happens if you let only P and V change? What about P and n? P and T? Graph these things and become more comfortable with handling equations. For the MCAT, and life in general, it does you little good to know the standard form of a linear relationship or that the whole thing is called a linear relationship. But fiddling with the behavior of these equations a bit by yourself is invaluable - and seeing what happens when you let some things change and others remain the same lets you see what statements any given equation is making.

This should if anything be the way you approach any equation: It describes a series of relationships. Isolate them individually to understand what is really being said by an equation.

Also, the relationship being described in this question is technically Charles's law. This isn't necessarily worth memorizing - my personal approach is to just take the standard ideal gas law and look at what stays the same and what changes in any given problem, and use a modified version accordingly (realizing that everything that stays the same is effectively a single constant).
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