Question #a0ccf
2 Answers
The student had to use 3.2 L of methanol.
Explanation:
Step 1. Calculate the molality of the solution.
The formula for calculating freezing point depression is
#color(blue)(bar(ul(|color(white)(a/a)ΔT_"f" = K_fmcolor(white)(a/a)|)))" "#
where
We can rearrange the above formula to get
#m = (ΔT_"f")/ K_"f"#
In this problem,
∴
Step 2. Calculate the mass of the solution.
When we mix two liquids, we cannot assume that their volumes are additive,
For example, 1 L of methanol + 2 L of water gives 3.1 L of solution.
This complicates the calculation.
(a) Calculate the percent composition of the solution.
(b) Look up the density of the solution.
The density of 34.0 % aqueous MeOH is 0.945 g/mL.
(c) Calculate the mass of the solution.
Step 3. Calculate the mass of methanol
Step 4. Calculate the volume of methanol
The student had to use 3.2 L of methanol.
Explanation:
This question appears to assume (incorrectly) that the volumes on mixing of liquids are additive.
We can use only two significant figures, so we might get away with it.
Step 1. Calculate the molality of the solution.
#color(blue)(bar(ul(|color(white)(a/a)ΔT_"f" = K_fmcolor(white)(a/a)|)))" "#
Step 2. Calculate the volumes of methanol and water in the above solution.
Step 3. Calculate the volume of methanol in 8.0 L of solution
It worked! We get the correct answer with much less work.