If you want to lower waters freezing point by 23°C by adding ${MgF}_{2}$ $"MgF"_2$ , what must the molality of the solution be?

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Answer 1 · 2018-06-04T23:06:35

$4.12 molal$ $"4.12 molal"$

Change in freezing point is given by

${ΔT = - iK}_{f} m$ $"ΔT = – iK"_f"m"$

Where

$i =$ $"i ="$ Van’t Hoff factor ( $3$ $3$ for ${MgF}_{2}$ $"MgF"_2$ )
$K_{f} =$ $"K"_f ="$ Freezing point constant ( $1.86°C/molal$ $"1.86°C/molal"$ for water)
$m =$ $"m ="$ Molality of solution

$m = \frac{ΔT}{{- iK}_{f}}$ $"m" = "ΔT"/("– iK"_f)$

$m = \frac{- 23°C}{- 3 \times 1.86°C/molal}$ $color(white)("m") = "– 23°C"/"– 3 × 1.86°C/molal"$

$color(white)("m") = 4.12\ "molal"$

If you want to lower waters freezing point by 23°C by adding "MgF"_2MgF2"MgF"_2, what must the molality of the solution be?