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We know the $T_f = 0°C$ for water.

Hence, $DeltaT_f = 2.25$. Moreover, nonvolatile solutes' van't Hoff factor is generally 1.

And, $DeltaT_f = iK_fm$

We'll do our calculation given those variables, as such

$2.25°C = 1 * (1.86°C)/(m) * m$
$therefore m = 1.21m$

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