How does molecular weight affect freezing point?

1 Answer
Aug 24, 2016

Answer:

If everything else is kept constant, a compound with a higher molecular mass will have a smaller effect on the freezing point.

Explanation:

The formula for calculating freezing point depression is

#color(blue)(bar(ul(|color(white)(a/a) ΔT_"f" = K_fbcolor(white)(a/a)|)))" "#

where

#ΔT_"f"# is the decrease in freezing point
#K_"f"# is the molal freezing point depression constant
#"b"# is the molal concentration of the solution.

#"Molal concentration" = "moles of solute"/"kilograms of solvent"#

Let's rewrite this formula in symbols as

#b = n/m_1#

Then

#color(blue)(bar(ul(|color(white)(a/a)ΔT_"f" = K_fn/m_1color(white)(a/a)|)))" "#

Now,

#"moles" = "mass"/"molar mass"#, or #n = m/M#

We can rearrange the formula to give

#color(blue)(bar(ul(|color(white)(a/a)ΔT_"f" = K_fm/(Mm_1)color(white)(a/a)|)))" "#

If #K_f, m#, and #m_1# are held constant, then

#color(blue)(bar(ul(|color(white)(a/a)ΔT_"f" ∝ 1/Mcolor(white)(a/a)|)))" "#

Thus, as the molar mass increases, the freezing point depression decreases.

That is, increasing the molar (or molecular) mass will have a smaller effect on the freezing point.