Question

Air contains nitrogen and oxygen in the volume ratio of 4:1 .the average molecular weight of air is?

Answer 1

Molecular mass of O2    -     32 amu
Molecular mass of N2.   -     28 amu  
(This means that if you take 32 grams of oxygen or 28 grams of nitrogen it will contain approximately 6.022 X 10 to the power 23 lmolecules of it)

                  amu ----- atomic mass unit

Let the mass of oxygen taken be Y grams then according to the question the mass of nitrogen is 4Y grams. Now to calculate the no of particles we divide the mass taken by amu.

For oxygen we get ,  Y/32.               ------- result 1
For nitrogen we get,  4Y/28.           --------result 2

Now the ratio of no. of particles for oxygen by nitrogen is  

              Result 1   /    Result 2.                         which is equals to

            Y/32  into  4Y/28. 

By solving we get the ratio 

                7/32.   -------- required ratio

Answer 2

Explanation:

average molecular mass=wt. of N2×given ratio +wt.of O2 ×given ratio÷total ratio

=>28×4+32×1÷5

=>112+32÷5

=>144÷5

=>28.8