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प्रश्न
Write the formula to determine the molar mass of a solute using freezing point depresssion method.
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उत्तर
Depression of freezing point and molar mass of the solute:
When W2 gram of solute with molar mass M2 is dissolved in W1 gram of solvent, the molality m is given by
`ΔT_f ∝ m`
`m="Mass of solute in kg"/("Molar mass of solue in kg "×" Mass of solvent in kg")`
`m=W_2/(M_2*W_1)`
∴ The molar mass of a solute using freezing depression method is givenby,
`ΔT_f ∝ m`
`ΔT_f = k_f . ("W"_2)/("M"_2) xx 1000/"W"_1`
Where `K_f` - Cryoscopic constant
`"W"_2` - mass of solute
`"W"_1` - mass of solvent
`"M"_2` - molecular mass of non-volatile solute
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| Henna is investigating the melting point of different salt solutions. She makes a salt solution using 10 mL of water with a known mass of NaCl salt. She puts the salt solution into a freezer and leaves it to freeze. She takes the frozen salt solution out of the freezer and measures the temperature when the frozen salt solution melts. She repeats each experiment. |
| S.No | Mass of the salt used in g |
Melting point in °C | |
| Readings Set 1 | Reading Set 2 | ||
| 1 | 0.3 | -1.9 | -1.9 |
| 2 | 0.4 | -2.5 | -2.6 |
| 3 | 0.5 | -3.0 | -5.5 |
| 4 | 0.6 | -3.8 | -3.8 |
| 5 | 0.8 | -5.1 | -5.0 |
| 6 | 1.0 | -6.4 | -6.3 |
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