Question

A certain substance melts at a temperature of . But if a sample of is prepared with of urea dissolved in it, the sample is found to have a melting point of instead. Calculate the molal freezing point depression constant of . Round your answer to significant digits.

Answer 1

Answer:

2.2 °C/m

Explanation:

It seems the question is incomplete. However, this problem has been found in a web search, with values as follow:

" A certain substance X melts at a temperature of -9.9 °C. But if a 350 g sample of X is prepared with 31.8 g of urea (CH₄N₂O) dissolved in it, the sample is found to have a melting point of -13.2°C instead. Calculate the molal freezing point depression constant of X. Round your answer to 2 significant digits. "

So we use the formula for freezing point depression:

• ΔTf = Kf * m

In this case, ΔTf = 13.2 - 9.9 = 3.3°C

m is the molality (moles solute/kg solvent)

• 350 g X ⇒ 350/1000 = 0.35 kg X

• 31.8 g Urea ÷ 60 g/mol = 0.53 mol Urea

Molality = 0.53 / 0.35 = 1.51 m

So now we have all the required data to solve for Kf:

• ΔTf = Kf * m

• 3.3 °C = Kf * 1.51 m

• Kf = 2.2 °C/m