[Faisal]

In my textbook(Zumdahl 7e)...it calculates Kp and doesn't show any units and it also says K(Kc) doesn't have units?Does that mean Kp doesn't have units?But in my exam they asked us to specify the units of Kp?

[sjb]

At a high school level, whether an equilibrium constant has units is a function of the actual equilibrium.

If the number of moles of product and reactant are the same, such as in the generic alcohol + acid <-> ester + water, then K_c will not have any units, but something like BaseH⁺_(aq) <-> Base_(aq) + H⁺_(aq) will have units.

Similarly, a reaction like H₂ + Cl₂ <-> 2HCl will not have units for K_p, but CaCO₃ <-> CaO + CO₂ will.

(double headed arrow meant to represent the equilibrium arrow)

I assume, from the area you've posted the topic in, that this is the more appropriate situation and sort of answer you're looking for.

At higher levels you learn about activity coefficients and reaction quotients, which are factors multiplied to concentrations or similar to allow for effective concentrations or pressures (a typical example being in concentrated solutions of strong acid, dissociation is not complete, so pH != -log[H⁺]), these too have the effect of giving no units for any equilibrium constant, K.

S

[AWK]

Generally, units of K are omitted. This comes from problem with logarithmic equation. But for calculating K you should use specific units of pressure or concnetration. Then dividing by unit pressure  (10⁵ Pa) or concentration (1 mol/dm³) you will get numbers. Using this numbers in K you will get K without units

[Faisal]

ohk thank you....my shool must be really dumb to ask such a higher level question.So if the # of moles are different...what is the units then?I put my answer as "atm"...is that correct?

[sjb]

ohk thank you....my shool must be really dumb to ask such a higher level question.So if the # of moles are different...what is the units then?I put my answer as "atm"...is that correct?

Without knowledge of the question, it's impossible to say whether atm is the correct unit for K_p. It would only be correct if the reaction in question were one like the decomposition of calcium carbonate to calcium oxide and carbon dioxide in my earlier example. If the reaction in question was, for instance, Ni(CO)_4(s) <-> Ni_(s) + 4 CO_(g) what units would you answer?

S