Question

The given graph represents the variation of Z (compressibility factor) vs. P for three real gases A, B and C. Identify the correct statement

• A. For the gas A, $a=0$ and its dependence on P is linear at all pressure
• B. For the gas B, $b=0$ and its dependence on P is linear at all pressure
• C. For the gas C, which is typical real gas for which $a\ne 0,b\ne 0$. By knowing the minima and the point of intersection with $Z=1,a$ and b can be calculated
• D. At high pressure, the slope is positive for all real gases

Answer 1

Answer: (A), (B), (D)

The correct options are
A For the gas A, $a=0$ and its dependence on P is linear at all pressure
B For the gas B, $b=0$ and its dependence on P is linear at all pressure
D At high pressure, the slope is positive for all real gases

According to a real gas equation,
The constants 'a' and 'b': Van der Waals constant for attraction 'a' and volume 'b' are characteristic constants for a given gas.
(i)The 'a' values for a given gas are a measure of intermolecular forces of attraction. More are the intermolecular forces of attraction, more will be the value of a.
(ii)For a given gas van der Waals constant of attraction 'a' is always greater than van der Waals constant of volume 'b'.
(iii)The gas having a higher value of 'a' can be liquefied easily and therefore $H_2$ and He is not liquefied easily.
According to this, for gas A (Z>1), a=0 and its dependence on P is linear at all pressure and for gas B (Z<1), b=0 and its dependence on P are linear at all pressure.
Also,
At high pressure, the slope is positive for all real gases.