Effusion of a 1:1 mixture of two gases,represented by unshaded and shaded spheres in the diagram below,through a small pinhole produces the result shown below.The shaded spheres have a molecular mass of 20 amu.Which gas molecules have the higher average speed and what is the molecular mass of the unshaded molecules?
A)Unshaded molecules have higher average speed and molecular mass = 14 amu
B)Unshaded molecules have higher average speed and molecular mass = 17 amu
C)Unshaded molecules have lower average speed and molecular mass = 24 amu
D)Unshaded molecules have lower average speed and molecular mass = 29 amu

Question

Effusion of a 1:1 mixture of two gases,represented by unshaded and shaded spheres in the diagram below,through a small pinhole produces the result shown below.The shaded spheres have a molecular mass of 20 amu.Which gas molecules have the higher average speed and what is the molecular mass of the unshaded molecules? 
A)Unshaded molecules have higher average speed and molecular mass = 14 amu 
B)Unshaded molecules have higher average speed and molecular mass = 17 amu 
C)Unshaded molecules have lower average speed and molecular mass = 24 amu 
D)Unshaded molecules have lower average speed and molecular mass = 29 amu

Quizplus · Accepted Answer

According to Graham's law of effusion, the rate of effusion of a gas is inversely proportional to the square root of its molar mass. Since both gases are in a 1:1 mixture, they will have the same rate of effusion. Therefore, the unshaded molecules must have a lower molar mass than the shaded ones in order to compensate for their higher average speed. The only choice that satisfies this is A, where the unshaded molecules have a molecular mass of 14 amu.

Effusion of a 1:1 Mixture of Two Gases,represented by Unshaded