# Molecular weight determination from freezing point depression

1. Which is the better solvent for molar mass determinations by freezing point depression, benzene or cyclohexane?
2. Which is the better method for molar mass determination, freezing point depression or boiling point elevation?

What is the correct approach for solving these?

1. In my opinion, resonance structures might be the key concept, but I don't know how it would help or hurt.

2. I think boiling has a decomposing effect that we must avoid, meaning freezing the compound(s) is preferable.

But, both of these statements are inadequate.

Molar freezing point depression
Benzene      5.12 °C/m
Cyclohexane  20.0 °C/m

Molar boiling point elevation
Benzene      2.53 °C/m
Cyclohexane  2.79 °C/m


I would base my argument on the constants, instead of waving hands.

• What about the second question? – Pınar Mar 28 '15 at 22:02
• Which is better, 20.0 °C/m or 2.79 °C/m? – LDC3 Mar 28 '15 at 22:32
• I don't really understand the "better" concept here. – Pınar Mar 29 '15 at 0:23
• Better in the sense of determining the molecular mass of the compound from using either the boiling point or the freezing point. If you took a solution and divided in 2. On one you determine the freezing point depression and with the other you determine the boiling point elevation. If your thermometer is off by 0.2°C, which method would give you the closer value? – LDC3 Mar 29 '15 at 1:24
• what is m in °C/m stands for? minute or molar – Eka May 28 '15 at 8:16