Question

How does molecular weight affect freezing point?

Answer 1

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.