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Question

When $$45\ gm$$ solute is dissolved in $$600\ gm$$ water freezing point lower by $$2.2\ K$$, calculate molar mass of the solute.

 $$[K_{f} = 1.86\ K\ kg\ mol^{-1}]$$


A
63.4 gm
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B
80 gm
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C
90 gm
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D
21 gm
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Solution

The correct option is A $$63.4\ gm$$
$$\Delta T_f = K_f\times m$$
$$m = molality$$

$$m = \dfrac{45\times 1000}{\text{Molar mass of solute} \times 600}$$

Putting the value of m in the equation:
$$\Delta T_f = 1.86\times m$$
$$2.2 = 1.86\times \dfrac{75}{\text{Molar mass}}$$

Molar mass of solute $$= \dfrac{1.86\times 75}{2.2}$$

                                      $$=63.4\ gm$$

Hence, option A is correct.

Chemistry

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