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Leokris [45]
3 years ago
13

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.
Chemistry
1 answer:
Harrizon [31]3 years ago
7 0

Answer:

27.60 g urea

Explanation:

The <em>freezing-point depression</em> is expressed by the formula:

  • ΔT= Kf * m

In this case,

  • ΔT = 5.6 - (-0.9) = 6.5 °C
  • Kf = 7.78 °C kg·mol⁻¹

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

First we<u> calculate the molality</u>:

  • 6.5 °C = 7.78 °C kg·mol⁻¹ * m
  • m = 0.84 m

Now we<u> calculate the moles of ure</u>a that were dissolved:

550 g X ⇒ 550 / 1000 = 0.550 kg X

  • 0.84 m = mol Urea / 0.550 kg X
  • mol Urea = 0.46 mol

Finally we <u>calculate the mass of urea</u>, using its molecular weight:

  • 0.46 mol * 60.06 g/mol = 27.60 g urea

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