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,

#DeltaT_"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.