Science Buddies: "Ask an Expert"

I am following the experment "Chemistry of Ice-Cream Making: Lowering the Freezing Point of Water". I completely understand the procedure until I get to #8. Why do I need to calculate the molalities of the NaCl and sucrose solutions? Why can't I just use the temperature readings that I got while I was doing the experment? Please help! Right now it just seems like alot of extra work for no reason (

Thanks!

Step 8 allows you to determine basically how much impurity you are adding to water and plot that against how much that changed the freezing point. Molality is better than just the number of grams you added, because the same weight of NaCl and sucrose will change the freezing properties differently...for example, 5g of NaCl and 5g of sucrose will have different molality because the gram molecular weight is completely different. The experiment goes on to say at step 11 to plot molality vs delta temp.

Hope this clarifies things. Ask if you have any other questions.

thank you for your reply. I understand that the molecular weight of each substance is different, but in the experement it doesn't say to use the same amount (in grams) of salt and surgar. There are completely different amounts stated. So I still don't see how the molality is relevent. Guess next year I'll pick one that isn't 2 grade levels above where I am!

Hello:

Molality is relevant because that's what changes the freezing point, not the substance itself. This experiment shows that. Also note that the graph you create with your data is freezing point depression vs. molality:

For each of your NaCl and sucrose solutions, graph the average amount of freezing point depression (ΔT, y-axis) vs. molality.

Keith