Serum osmolarity vs. serum osmolality?

[Jedi of Zen]

I'm trying to figure out the difference in definitions between serum osmolarity and serum osmolality.

My textbook (Fundamentals of Nursing, 6th edition, by Potter & Perry) states that normal serum osmolarity in the blood is 280 - 295 mOsm/kg.

However, every website that I have ever looked up on the matter states that serum osmolality is 280 - 295 mOsm/kg.

Does that mean that the 2 terms are exactly synonymous? If not, then which is wrong - my book or all of those other websites? And why?

Thanks!