Question

A certain substance X has a normal boiling point of 101.5°C and a molar boiling point elevation constant Kb = 1.49 °C/m. A solution is prepared by dissolving some urea (NH2)2CO in 200 g of X. This solution boils at 104.4°C. What is the mass of urea that was dissolved in the solution? A. 23.4 g B. 6.48 g C. 584 g D. 9.73 g E. 1.95 g

Question 10 of 10 1.0 Points The normal freezing point of a certain liquid Y is -8.70°C, but when 24.17g of urea (NH2)2CO are dissolved in 550 g of Y, it is found that the solution freezes at -13.7°C instead. Use this information to calculate the molal freezing point depression constant Kf of substance Y. A. 0.15°C/m B. 3.7°C/m C. 31°C/m D. 6.8°C/m E. 5.0°C/m

Answer 1

9.

We know that ΔT _b = K_bx m
Where

ΔT _b= elevation in boiling point

        = boiling point of solution - boilinging point of pure solvent

        = 104.4 - 101.5 ^oC

= 2.9 oC

K_b = elevation in boiling point constant of water = 1.49 ^oC/m

m = molality of the solution

    = ( mass / Molar mass ) / weight of the solvent in Kg

    = (m / 60) / 0.20 kg

    = m / 12

Plug the values we get

2.9 = 1.49x(m/12)

m = 23.4 g/mol

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(10)

We know that ΔT _f = K_f x m
Where

ΔT _f = depression in freezing point

        = freezing point of pure solvent – freezing point of solution

        = -8.70 - (-13.7) oC

= 5 oC

K _f = depression in freezing constant = ?

m = molality of the solution

    = ( mass / Molar mass ) / weight of the solvent in Kg

= ( 24.17 / 60) / 0.550

= 0.732

Plug the values we get

Kf = 5.0 / 0.732

= 6.8 oC/m