Science Buddies: "Ask an Expert"

I was interested to know how the number of grams for the Chemistry of Ice-Cream Making: Lowering the Fressing Point of Water experiment were chosen. The test liquids are listed as NaCl - 2.9g then doubled to 5.8g and 11.7g and sugar - 17.1g doubled to 32.2g and 68.5g. Is there a formula behind or explanation behind these numbers being used? If we wanted to try it with calcium chloride to compare how would we determine how much in grams we would need to use? Thank you.

Hello abmjr27,

The weights of NaCl and sugar listed in the project procedure were chosen to give the same molal concentrations (0.5, 1 and 2 mol/Kg) for both compounds. For example, when you complete Tables 1 and 2 in the procedure you’ll see that 2.9 g NaCl in 100 mL of water is the same molality (0.5 mol/Kg) as 17.1 g sucrose in 100 mL of water. And likewise for the other weights of NaCl and sugar.

If you want to try calcium chloride in your experiment, you should use the same concentrations used for the sugar and NaCl. The molal concentration is the number of moles per kilogram of solvent. And the molecular weight of calcium chloride is 110.98 g/mol. So, if you want 100 mL (or 0.1 Kg) of a 0.5 molal solution you need 5.55 grams of calcium chloride.

I hope this helps. Please post again if you have more questions.

A. Norman

Thank you, that makes complete sense now.