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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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A sodium ion, Na+, with a charge of 1.6×10−19C and a chloride ion, Cl− , with charge of −1.6×10−19C, are separated by a distance
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Answer:

W\geq 2.1x10^{-19}J

Explanation:

Due to Coulomb´s law electric force can be described by the formula F=K\frac{q_{1}.q_{2}}{r^{2}}, where K is the Coulomb´s constant (9x10^{9} N\frac{m^{2} }{C^{2} }), q_{1}= Charge 1 (Na+ in this case), q_{2} is the charge 2 (Cl-) and r is the distance between both charges.

Work made by a force is W=F.d and total work produced is the change in energy between final and initial state. this is W=W_{f} -W_{i}.

so we have W=W_{f} -W_{i} =(K\frac{q_{(Na+)}q_{(Cl-)}rf}{r_{f} ^{2}})-(K\frac{q_{(Na+)}q_{(Cl-)}ri}{r_{i} ^{2}})=Kq_{(Na+)}q_{(Cl-)[\frac{1}{{r_{f}}} -\frac{1}{{r_{i}}}]

Given that ri= 1.1nm= 1.1x10^{-9}m and rf= infinite distance

W=(9x10^{9})(1.6x10^{-19})(-1.6x10^{-19})[\frac{1}{\alpha }-\frac{1}{(1.1x10^{-9})}]=2.1x10^{-19}J

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