in the following experiment, the molar mass of an unknown solid, which is nonvol

Question

in the following experiment, the molar mass of an unknown solid, which is nonvolatile and a nonelectrolyte, is to be determined by the freezing point depression method. The pure solvent used in the experiment freezes at 10 degrees celcius and has a known molal freezing point depression constant Kf.

1.)

a.) briefly and clearly explain how the differences between the two graphs in can be used to obtain information needed to calculate the molar mass of the unknown solid. ( 60 words or less, using complete sentences)

b.)Calculation in the percentage error in a student's result, if the student obtains a value of 126 g mol -1 for the molar mass of the solid when the actaul value is 120.15 g mol-1

Explanation / Answer

Depression in freezing point and calculation of molar mass of unknown

first plot graph of temperature vs time for pure solvent. Find freezing point from graph

second plot temperature vs time graph for solution with unknown dissolved in solvent. Determine freezing point of the solution.

Now from the two graphs

depression in freezing point = freezing point of solvent - freezing point of solution = dTf

Using relation,

dTf = i.Kf.m

with known Kf

m = molality of solution can be calculated

molarlity = moles of solute/kg of solvent

mass of solvent taken is known

calculate moles

moles of unknown = mass of unknown/molar mass of unknown

knowing mass of unknown taken in grams, molar mass in g/mol can be clauculated.

--

Experimental molar mass = 126 g/mol

Actual molar mass = 120.15 g/mol

Percent error = (126 - 120.15) x 100/120.15 = 4.87%

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.