What weight of oxygen gas will contain the same number of molecules as 56 g of nitrogen gas (O = 16 u; N = 14 u)

 mole = given weight / molecular weight = no. of particles / 6.022 * 10²³

no. of particles = given weight / molecular weight * (6.022 * 10²³⁾

so, 

no. of particles in 56g of N₂ gas is

56/(14 * 2) * 6.022 * 10^23

=12.044 * 10 ^23

let the no. of o2 molecules be x

so,

weight of o2 / molecular weight * {6.022 *10^23} = no. of particles 

=> x / (16 * 2) *(6.022 * 10^23) = 12.044 * 10^23

x = 64 g

ANS. 64g