The mass of a balloon is 0.02g before it is filled with air. After it is blown up with air the mass is 0.04g. Assuming oxygen makes up 21% of air, calculate the number of moles of oxygen in the balloon.

After the balloon has been blown up with air it increased its size by 0.02 g (Final mass - initial mass = mass of the air).We are told that of oxygen takes up 21% of air. Therefore, we need to work out 21% of the mass of the air in the balloon (0.02g) 0.21 x 0.02 g = 0.0042 g of oxygen in the balloonNow, we can work out how many moles are there in 0.0042 g from the mole equation Therefore,n = 0.0042 g / Mr of O₂ n = 0.0042 g / 16 x 2 g per moln = 0.00013125 molesNOTE: We have to remember that oxygen in air always exists as diatomic molecule (O₂) because it's more stable. Therefore, we need to multiply Mr of Oxygen by 2.