Answer: norfenefrine
Explanation:
Depression in freezing point is given by:
= Depression in freezing point
i= vant hoff factor = 1 (for non electrolyte)
= freezing point constant =
m= molality
Weight of solvent (ethanol)=
Molar mass of unknown non electrolyte = M g/mol
Mass of unknown non electrolyte added = 82 mg = 0.082 g (1g=1000mg)
The molecular mass of norfenefrine is near to 154 and thus the identity of the white powder is norfenefrine .
The depression in freezing point is related to molality of solution as follows:
Here, is freezing point depression constant, for cyclohexane it is equal to 20°C kg/mol.
The value of freezing-point depression is 2.5 °C, molality can be calculated as follows:
Molality is defined as number of moles of solute in 1 kg of solvent.
Here, solvent is cyclohexane, its volume is given 20.0 mL and density is 0.779 g/mol thus, mass of cyclohexane can be calculated as follows:
Converting this into kg,
Thus, 15.58 g will be 0.01558 kg.
Now, number of moles of unknown solute is related to its mass and molar mass as follows:
Putting the values of mass of solute which is 0.2436 or 0.0002436 kg
Now, expression for molality of solution is:
Putting all the values,
Or,
On rearranging,
Or,
Therefore, molar mass of unknown sample is 125 g/mol