# How many grams of NH_4OH must be dissolved in 75 g of water to lower the freezing point -3.0°C?

##### 1 Answer
Jun 1, 2018

Yielding to Michael's practical advice, although if this was the true problem your professor made (and often paper chemistry is impractical), the solution is below.

Recall,

$\Delta {T}_{\text{f" = iK_"f}} m$

where,

K_"f" = (1.86"C"°)/m

m = "mol"/("kg"_(H_2O))

$i = 2$ (assuming no ion pairing)

Hence, we need,

n = (DeltaT_"f" * "kg"_(H_2O))/(iK_"F") approx 6.05*10^-2"mol" * (35"g")/(NH_4OH) approx 2.1"g"

of that compound would theoretically lower the freezing point by the magnitude described.