Gen Chem Questions

[happyfellow]

Here's a couple seemingly straight-forward questions that are giving me trouble:

What is the density of air at STP?
A) 0.95 g/L
B) 1.29 g/L
C) 2.11 g/L
D) 3.67 g/L

Answer is B. Not sure how to approach it. Maybe this is just a value I should know?

For a 2.0 M aqueous salt solution with a density of 1.05 g/mL that is 9.17% solute by mass, what is the approximate molality.
A) 1.0 m
B) 1.9 m
C) 2.1 m
D) 4.2 m

Answer is C. Once again, not sure how to approach it.

Thanks in advance.

-Happy

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[novawildcat]

B.) just simply use the definition of molality= mol solute/kg SOLVENT


make your life easy by choosing easy numbers. Use 1000 mL. If you have 1 L of the solution, then you have 2 moles of the solute. Multiply the density by 1L to give how much of the soln you have in grams. 1000 x 1.05= 1050 g soln. 9.17% of that by mass comes from the solute. Just use rough math. 9.17% of 1050 is about 90 (a bit more though). Subtract 90 from 1050. You have about 960 g of solvent. Just plug into the definition of molal. 2 moles solute/.96 kg solvent which you know is slightly greater than 2.0. So it's C.

 

[loveoforganic]

(80% Nitrogen)(28 g/mol) / (1 mol / 22.4 L) + (20% Oxygen)(32 g/mol) / (1 mol / 22.4 L) = 1 + 0.29 = 1.29 g / L

This is assuming you know air is ~80% nitrogen and 20% oxygen. Pretty common knowledge, but I don't think it's part of MCAT material really...

 

[DendWrite]

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