Molar Mass Determination by Depression of the Freezing Point Measured Freezing P

Question

Molar Mass Determination by Depression of the Freezing Point Measured Freezing Point of Pure Water -013 degree C Finding the Freezing Point of a Solution of Liquid Unknown Target mass of solute (Calculated based on the parameter in the instructions) 10.00 g Unknown # 38 Q Actual mass of solute used 10,138 g Trial I Freezing point of solution (observed) -2.4 degree C Mass of solution 150,71 g Trial II Calculations: Trial I Freezing point depression -2,1 C degree Molality of unknown solution, m_a molal Mass of solution g Mass of solute g Mass of solvent (water) 140.572 g Moles of solute mol Molar mass of unknown g/mol

Explanation / Answer

Let's remember:

molality = moles of solute / kg of solvent

In this case, we don't have the exact moles of solute (because we need its molar mass to calculate moles, so we'll leave molality as a function of the molar mass MM):

molality (m) = grams of solute / (kg solvent * molar mass of solute)

m = 10.138g/(0.140572kg*MM)

m = (72.1196/MM) molal

The mass of the solution = mass of solute + mass of solvent

Mass of solution = 10.138g + 140.572g

Mass of solution = 150.71g

The mass of solute is the one you specified in the first part

Mass of solute = 10.138g

To calculate moles of solute we would need the molar mass MM, so we'll leave this as a function of MM too (like molality):

moles of solute = grams of solute / molar mass of solute

moles of solute = (10.138/MM) mol

To calculate the molar mass of the unknown substance we use the freezing point depression formula:

T = Kf m

We have T (2.1°C) , a constant Kf (1.86°C/m) and a molality expression:

(2.1°C) = (1.86°C/m)(72.1196/MM)m

Let's solve for MM:

MM = (1.86 °C/m)*(72.1196/MM)/(2.1°C)

MM = 63.87 g/mol

Now that we have molar mass, we can calculate molality and moles of solute:

m = (72.1196/MM) molal

m= (72.1196/63.87) molal

m = 1.129 molal

moles of solute = (10.138/MM) mol

moles of solute = (10.138/63.87) mol

moles of solute = 0.1587 mol

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