Finding the weight of Nitrogen in Air

[thevinciz]

Homework Statement

Air consists of a approximately 78% Nitrogen in volume,21% Oxygen and 1% Argon. The atomic weight of Nitrogen is 14, that of Oxygen 16 and that of Argon 40

(1) Nitrogen occupies air in weight by

(a) 76% (b) 77% (c) 78% (d) 79%

Homework Equations

The Attempt at a Solution

I have Nitrogen : 0.78L
Oxygen : 0.21L
Ar : 0.01L

And I used PV=nRT to find mass of each of gases
So I got mass of Argon = 0.4P/RT
mass of Nitrogen = 21.84P/RT
mass of Oxygen = 6.72P/RT
so weight of Nitrogen in air is 21.84/28.96 = 0.754, change into percent = 75.4%
but it's not the choices. What did I miss?

 

[DrClaude]

so weight of Nitrogen in air is 21.84/28.96 = 0.754, change into percent = 75.4%
but it's not the choices. What did I miss?

You don't seem to have missed anything. I would round up and choose 76% as the best answer.

 

[mjc123]

It's probably a question of rounding, how many decimal places you take etc. E.g. see Table 1 in http://www.nptel.ac.in/courses/123106002/MODULE%20-%20I/Lecture%201.pdf. It's very close to 75.5%, so it could go 75 or 76. Using the numbers (and precision) you are given, I agree with your answer.

 

[thevinciz]

Thank you very much everyone