Table of Contents

• Prequiz...
• 1: Osmosis = Only water can pass
• 2: Putting the Oz into Osmolarity
• 3: Figuring osmolarity
• 4: Adding osmolarities
• 5: Which direction does water flow?
• 6: And how fast?
• 7: More problem types...
• 8: Equilibrium
• 9: Sharks, alligators, and goldfish

Figuring osmolarity

if you know molarity

If you need to figure out the osmolarity of a solution and you're given the molar concentration, its pretty simple,
just multiply the molarity by however many "pieces" the solute dissociates into.

If you know amount of solute

What if you just know the amount of a solute that was added? For example: what is the approximate osmolarity of a can of coca cola? According to the can which just happens to be sitting next to me, there are 37g of sugar in 12oz of classic coke. (Remember we've calculated before that sugar has a molecular weight of 180g). Hmm, time to crank out some conversions:

First, grams to moles: 37g * 1 mole/180g = 0.205 moles

Next, ounces to liters: 12 oz * 1 liter / 33.8 oz = 0.36 liters

Finally, we know that sugar does not break apart, so 1M = 1 OsM.

Now that everything is in the right units, we just need to divide moles by liters to get the osmolarity:

0.205 moles / 0.36 liters = 0.57 OsM.

Think about that next time you have a coke and a Krispy Kreme for breakfast!