How do you calculate the following to determine if they are hypotonic, isotonic, or hypertonic?

Question

a. An ophthalmic solution containing 40 mg/mL of cromolyn sodium and 0.01% of benzalkonium chloride in purified water

b. A parenteral infusion containing 20 % (w/v) of mannitol

c. A 500-mL large-volume parenteral containing D5W (5% w/v of anhydrous dextrose in sterile water for injection)

(Tips on how to know what each question and similar such questions are asking for would be appreciated)

Answer 1

Before trying to provide an answer, it should be pointed out that tonicity (isotonic, hypotonic, hypertonic) depends on the cell of interest. We will assume we are considering a typical mammalian cell. Next consideration, is what is considered normal tonicity. We can take that as approximately 250 - 300 mOsM. With these considerations in mind, we proceed as follows:

a) I believe we'd need the molar masses for the ingredients so that we could calculate osmoles.

b) molar mass mannitol = 182 g / mole

20% w/v = 20 g mannitol / 100 mls = 200 g / L

Since mannitol does not dissociate or ionize in solution, 1 mol = 1 Osmole

200 g / L x 1 mole / 182 g = 1.1 mole/L = 1.1 OsM = HYPERTONIC

c) 5% dextrose is ISOTONIC by definition. To calculate, we have...

molar mass dextrose = 180 g/mole

5% dextrose = 5 g / 100 ml = 50 g / L

50 g / L x 1 mol / 180 g = 0.278 M and 1 mol dextrose = 1 Osmole since it doesn't ionize

0.278 OsM = 278 mOsM = ISOTONIC