Partial Pressure vs. Brackets

[skyeblee2F]

For problem 11.13 in the 6th edition, in the answer booklet, the quotients for (a) and (c) are written in terms of partial pressure (so P with the subscript). The answer for (b), however, is written with the molecules placed inside brackets. Is there a reason why (b) is formatted differently? Or is this merely showing that both formats are accepted and interchangeable?

[Maddy Mackenzie]

In a and c, the substances included in the equation are all gases so you do the partial pressure but for b they are aqueous so it is better to do the concentration

[Kevin ODonnell 2B]

Square brackets [] denotes concentration and it is only used for Kc. Gases (Kp) however use parentheses (). So no they are not interchangeable. This is because Kc is used as an equilibrium constant, while Kp stands for partial pressure.