How does molecular weight affect freezing point?

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Answer 1 · 2016-08-24T15:49:53

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

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.