molar mass of an unknown solid terms

purpose of experiment

to determine the molar mass of an unknown solid by measuring how much it lowers the freezing point of an unknown solvent (cyclohexane) when dissolved.

freezing point depression is a

colligative property

colligative property

depends only on the number of solute particles, not their identity

when a nonvolatile solute dissolves in a solvent

it lowers the freezing point because solute particles interfere with the solvent’s ability to form a solid lattice.

temperature difference between pure solvent and solution is

ΔTf = Kf × m

change in freezing point

ΔTf

molal freezing-point constant

Kf

molality = moles solute / kg solvent

m

freezing point of pure cyclohexane

Tf,pure​

Value = 8.0 degrees Celsius 

freezing-point constant

Kf

value = 20.0 degrees Celsius * kg / mol

density (if needed)

p

value = 0.7785 g/mL

equipment set up

dry, clean test tube or freezing tube assembly

cyclohexane (solvent)

unknown solid (solute)

thermometer probe or temperature sensor

stirring rod or wire loop

cooling bath (ice + salt or cold water bath)

ring stand + clamp

step 1

weigh the assembly

• record the mass of the test tube assembly alone

• add cyclohexane; weigh again to find mass of solvent

step 2

measure freezing point of pure cyclohexane

• place assembly in ice bath

• stir gently while recording temperature every 20-30 seconds 

• identify the plateau - where temperature stays nearly constant = freezing point

• record as Tf,pure

step 3

add unknown solid

• weigh ~ 0.2 - 0.3 g solid, record mass

• add it to the same cyclohexane (do not replace solvent)

• dissolve fully by warming gently (do not overheat)

step 4

measure freezing point of the solution

• repeat cooling process

• record time vs temperature until plateau forms

• record Tf,solution

step 5

repeat with more solute (trial 2)

• add more solid (known additional mass)

• redissolve and record new Tf,solution

step 6

determine ΔTf​ = Tf,pure ​− Tf,solution​ for each trial

step 7

calculate molality, moles, and molar mass

• use each ΔTf​ to find molality

• convert molality - moles solute using kg of solvent

• divide grams of solute by moles to find molar mass

• average both trials for final result

K f cyclohexane

20.0 degrees Celsius * kg / mol

constant

Tf,pure

8.0 degrees Celsius

constant 

some solute did not dissolve

molar mass too high

ΔTf smaller means molality is smaller 

cyclohexane evaporated

solution more concentrated

ΔTf larger means molar mass is too low 

accidentally used twice of the same solvent

ΔTf doubles

colligative property is proportional to solute amount 

solution ionized (electrolyte)

effective number of particles increases

ΔTf larger means apparent molar mass too low 

use the same sample for both trials

ensures the same solvent mass 

consistent results

eliminates extra error 

why cyclohexane instead of water

nonpolar, dissolves organic solids, has larger Kf for measurable ΔTf

find the plateau first instead of freezing point

it represents equilibrium between liquid and solid

true freezing point

type of property of freezing point depression

colligative property

if the solute were volatile

could evaporate with cyclohexane

unpredictable ΔTf

invalid result 

cooling curve has a

smooth temperature drop (liquid cooling)

flat plateau (freezing point)

then another drop (solid cooling)

cyclohexane is

flammable - keep away from flames 

where to dispose of solutions

organic liquid waste