# Temperature and mass density

#### jmb07

Average molecular mass of air is 29 u. What is the mass density of air at 1 atm and -50 degrees celsius??

This is what i have so far....

V=(NkT)/P so V= [(29u)(1.38E^-23)(-50 + 273)]/ 1 = 8.9 x10^-20

The mass is equal to 29u x (6.022 x 10^23) =1.75 x 10^25

density =m/v so = 1.96 x 10^44..........and that is wayyyy off! The correct answer is like around 1, 2, or 3....i need some help. What am i doing wrong?

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#### Mapes

Homework Helper
Gold Member
Try carrying your units through your equations. What does it mean to have a molecular mass of 29?

#### jmb07

uh...i know i could try to convert the 29u to (number of atoms)...but i dont have any molecular weight

#### Mapes

Homework Helper
Gold Member
29u tells you something about the relationship between mass (which is connected to density) and the number of moles (which is connected to the ideal gas law that you're using).

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