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A certain liquid X has a normal boiling point of 118.4 °C and a boiling point elevation constant K=2.40 °С kg-mol-1. A solution is prepared by dissolving some benzamide (C7H7NO) in 150g of X. This solution boils at 120.6 °C. Calculate the mass of C7H7NO that was dissolved.Be sure your answer is rounded to the correct number of significiant digits.

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Answer:

43.47 g

Explanation:

The boiling point elevation is described as:

  • ΔT = K * m

Where ΔT is the difference in boiling points: 120.6-118.4 = 2.2 °C

K is the boiling point elevation constant, K= 2.40 °C·kg·mol⁻¹

and m is the molality of the solution (molality = mol solute/kg solvent).

So first we calculate the molality of the solution:

  • ΔT = K * m
  • 2.2 °C = 2.40 °C·kg·mol⁻¹ * m
  • m=0.917 m

Now we calculate the moles of benzamide (C₇H₇NO, MW=315g/mol), using the given mass of the liquid X.

  • 150 g ⇒ 150/1000 = 0.150 kg
  • 0.917 m = molC₇H₇NO / 0.150kg
  • molC₇H₇NO = 0.138 mol

Finally we convert moles of C₇H₇NO into grams, using its molecular weight:

  • 0.138 molC₇H₇NO * 315g/mol = 43.47 g

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