Mean free path and Avogadro's number — NEET & JEE Questions

Question 1 · difficulty L1 · recall

Mean free path is the average distance travelled by a gas molecule between

• A. successive collisions
• B. successive wall reflections only
• C. two phase changes
• D. two chemical reactions

Question 2 · difficulty L2 · understanding

At fixed temperature, decreasing gas pressure generally causes the mean free path to

• A. increase
• B. decrease
• C. remain exactly constant
• D. become zero

Question 3 · difficulty L2 · understanding

Avogadro's number represents the number of constituent particles in

• A. one mole
• B. one kilogram
• C. one litre of every substance
• D. one atom

Question 4 · difficulty L2 · understanding

The numerical value of Avogadro's constant is approximately

• A. $6.02\times10^{23}\,mol^{-1}$
• B. $6.02\times10^{-23}\,mol^{-1}$
• C. $3.00\times10^8\,mol^{-1}$
• D. $1.38\times10^{-23}\,mol^{-1}$

Question 5 · difficulty L3 · application

How many molecules are present in 2 mol of an ideal gas?

• A. $1.204\times10^{24}$
• B. $6.02\times10^{23}$
• C. $3.01\times10^{23}$
• D. $2.00\times10^{23}$

Question 6 · difficulty L3 · application

A sample contains $3.01\times10^{23}$ molecules. How many moles is this approximately?

• A. 0.50 mol
• B. 1.0 mol
• C. 2.0 mol
• D. 3.01 mol

Question 7 · difficulty L3 · application

At constant temperature, if gas pressure is reduced to one-half and molecular diameter is unchanged, mean free path approximately becomes

• A. twice
• B. half
• C. four times
• D. unchanged

Question 8 · difficulty L3 · application

Which change would most directly increase mean free path in a gas at fixed pressure?

• A. Increasing temperature
• B. Increasing molecular diameter
• C. Increasing number density
• D. Compressing the gas

Question 9 · difficulty L4 · application

A gas has mean free path $\lambda$ at pressure P. At the same temperature its pressure becomes 4P. What is the new mean free path?

• A. $\lambda/4$
• B. $4\lambda$
• C. $\lambda/2$
• D. $2\lambda$

Question 10 · difficulty L4 · application

A 0.25 mol gas sample contains how many molecules approximately?

• A. $1.51\times10^{23}$
• B. $2.41\times10^{24}$
• C. $6.02\times10^{23}$
• D. $4.00\times10^{22}$