Chemistry MCQ Practice

The apparatus can be used to show the diffusion of gases.

 

 

Which pair of gases A and B would cause a fall in the water level at the right side of the U-shaped tube?

 

A)  A = carbon monoxide, B = nitrogen

 

B)  A = fluorine, B = argon

 

C)  A = nitrogen dioxide, B = chlorine

 

D)  A = oxygen, B = neon

 

 

Answer: D

 

- In a porous pot qns, a gas diffuses into the pot while another gas diffuses out i.e. A diffuses in and B diffuses out.

 

- If gas A diffuses in faster than gas B diffuses out → over time there will be more gas particles inside the pot → pressure in the pot increase.
- If gas A diffuses in slower than gas B diffuses out → over time there will be less gas particles inside the pot → pressure in the pot decrease.

 

- For the water level at the right side to fall, the pressure in the pot must decrease i.e. gas A must diffuse in slower than gas B diffuses out.
- We can hence deduce that gas A must have a large relative molecular mass (i.e. be heavier) than gas B.

 

Common mistake: students will put down the relative mass of nitrogen to be 14, oxygen to be 16, chlorine to be 35.5 and fluorine to be 19. Bear in mind that these gases exists as a diatomic molecule i.e. N₂, O₂, Cl₂, F₂.