What is the molality of a solution?

Question

What is the molality of a solution consisting of 2.67 mL of carbon tetrachloride (CCl4, d = 1.59 g/mL) in 79.7 mL of methylene chloride (CH2Cl2, d = 1.33 g/mL)?

Answer 1

Atomic weights: C=12 Cl=35.5 CCl4=154

Let carbon tetrachloride be called CT. Let methylene chloride be called MC

2.67mLCT/79.7mLMC x 1mLMC/1.33gMC x 1000gMC/1kgMC x 1.59gCT/1mLCT x 1molCT/154gCT = 0.260 molCT/kgMC which is molality.

Answer 2

The weight of the carbon tetrachloride is the product of the density times volume. This is 4.25 grams. Next, the number of moles has to be calculated. This is the weight divided by the molecular weight of CCl4. The molecular weight is the sum of the atomic weights. This all factors out to be 0.028 moles of CCl4. Molarity is defined as the number of moles per liter of solvent. If the solvent is CH2Cl2, then 79.7 ml is equivalent to .080 liters. .028 moles per .080 liters gives a molarity of .035 moles/liter.