Question 11

[mashbour8199]

Hey there,

Just for the record, what is the actual molecular weight of air? It says in the answer that it's 28 g/mol + 32 g/mol, but that can't be, because the total for that would be 60 g/mol, which is LARGER than 44 g/mol (CO2) which would ultimately make CO2 a wrong answer.

If anyone could please shed some light on this, that would be great! Thanks!

[mcat_premed3832]

Air is not a single compound and in order to estimate its molecular weight, you would not add the different MW's of the compounds that make up air. You would, instead, do something like this:

(0.78 )(28 g/mol)
+ (0.21)(32 g/mol)
+ (0.01)(avg molecular weight of all remaining components of air)

= the overall molecular weight of air

Thus the molecular weight of air's components (ie N2) must be multiplied by the fraction that it contributes to air (ie 0.78 ). Thus, in all likelihood, the overall molecular weight of air is somewhere between 28 and 32 (but closer to 28 since it is by far the largest component of air).

The above is MCAT level knowledge but if you want to see it done more precisely: www.engineeringtoolbox.com/molecular-mass-air-d_679.html