Question

If volume percentage of nitrogen in air is 78% then
the number of N2 molecules present in
10 L of air at STP is
(1) 1.25 NA
(2) 0.35 NA
(3) 0.18 Na
(4) 0.78 NA​

Answer 1

Answer0.35 Na

Explanation:

Taking out moles of Nitrogen in 10 L of air comes out to be 0.35 and moles * Na = no. Of molecules

78% = 7.8 L in 10L of air so

moles at STP = 7.8/22.4 = 0.35

Answer 2

answer : option (2) 0.35 NA

explanation : volume percentage of nitrogen in air = 78%

so, in 10L of air, volume of nitrogen gas = 78% of 10L = 7.8L

we know, no of moles in term of volume at STP is given as , mole = volume of gas/22.4L

so, no of moles of N2 at STP = 7.8/22.4

= 0.34821 ≈ 0.35

now, number of N2 molecules = no of moles of N2 × Avogadro's number

= 0.35 × NA

hence, option (2) is correct choice.