Question 2

Diagram below shows two balloons respectively filled with 500 cm³ of oxygen gas and 750 cm³ of ammonia gas at room condition. Find out which balloon is heavier.
[RAM: O, 16; N, 14; H, 1]
[Molar volume of gas: 24 dm³mol^-1]    

Answer:
Looking at the size of the balloons, you might assume Balloon B is heavier than Balloon A, but the key here is the mass of gas contained in each balloon.

Step 1:  The number of moles of each gas.

Balloon A
=  Volume of gas / Molar volume of gas
=  0.5 dm³ / 24 dm³mol^-1
=  0.021 mol


Balloon B
=  Volume of gas / Molar volume of gas
=  0.75 dm³ / 24 dm³mol^-1
=  0.031 mol


Step 2:  The mass of each gas.

Balloon A
=  Number of moles x RMM
=  0.021 mol x [16x2] gmol^-1
=  0.021 mol x 32 gmol^-1
=  0.672 g of oxygen gas


Balloon B
=  Number of moles x RMM
=  0.031 mol x [14 + 1x3] gmol^-1
=  0.031 mol x 17 gmol^-1
=  0.527 g of ammonia gas

Balloon A is heavier than Balloon B