JEE Advanced · Chemistry · 5. States of Matter
The given graph represent the variations of compressibility factor \((Z)=\frac{P V}{n R T}\) versus \(P\), for three real gases \(A, B\) and \(C\). Identify the only incorrect 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 neither \(a\) nor \(b=0\). By knowing the minima and point of the intersection, with \(Z=1, a\) and \(b\) can be calculated.
- D At high pressure, the slope is positive for all real gases.
Answer & Solution
Correct Answer
(B) For the gas \(B, b=0\) and its dependence on \(P\) is linear at all pressure.
Step-by-step Solution
Detailed explanation
From the graph it is clear that the value of \(Z\) decreases with increase of pressure. We can explain as follows on the basis of van der Waals' equation.
At high pressure, when ' \(P\) ' is large, \(V\) will be small and one can not ignore ' \(b\) ' in comparison to \(V\).
However, the term \(\frac{a}{V^2}\) may be considered negligible in comparison to ' \(p\) ' in van der Waals' equation
or
\(\left(p+\frac{a}{V^2}\right)(V-b) =n R T \)
\( P(V-b) =n R T \)
\( p V-p b =n R T \)
\( \frac{p V}{n R T} =1+\frac{p b}{n R T} \)
\( Z =1+\frac{p b}{n R T}\)
or
Here, \(Z\) is greater than one and it increases linearly with pressure. Hence, statement (b) is false.
At high pressure, when ' \(P\) ' is large, \(V\) will be small and one can not ignore ' \(b\) ' in comparison to \(V\).
However, the term \(\frac{a}{V^2}\) may be considered negligible in comparison to ' \(p\) ' in van der Waals' equation
or
\(\left(p+\frac{a}{V^2}\right)(V-b) =n R T \)
\( P(V-b) =n R T \)
\( p V-p b =n R T \)
\( \frac{p V}{n R T} =1+\frac{p b}{n R T} \)
\( Z =1+\frac{p b}{n R T}\)
or
Here, \(Z\) is greater than one and it increases linearly with pressure. Hence, statement (b) is false.
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