A certain substance X has a normal freezing point of 5.6 °C and a molal freezing point depression constant Kf-7.78 °C-kg·mol-1. A solution is prepared by dissolving some urea ((NH2)2CO) in 550. g of Χ. This solution freezes at-0.9 °C. Calculate the mass of urea that was dissolved. Be sure your answer has the correct number of significant digits.

27.60 g urea

Explanation:

The freezing-point depression is expressed by the formula:

In this case,

m is the molality of the urea solution in X (mol urea/kg of X)

First we calculate the molality:

Now we calculate the moles of urea that were dissolved:

550 g X ⇒ 550 / 1000 = 0.550 kg X

Finally we calculate the mass of urea, using its molecular weight:

in the analysis of systems that involve fluid flow, we frequently encounter the combination of properties u and pv.