Which of the following solvent pairs could be used in a solvent pair recrystallization (in general, not for this specific reacti
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Answer:
c. A and C have the same boiling point, but B has a lower one.
Explanation:
The freezing point depression is expressed as:
ΔT = kf×m
Where kf is the freezing point depression constant that depends the nature of the solvent and m is molality (mol of solute/kg of solvent)
Boiling point elevation is:
ΔT = kb×m
Where kb is the ebulloscopic constant of the solvent.
As kf, kb and mass of solvent is the same for A, B and C solutions the only difference will be in moles of solute.
<em>A </em>6,84g of sucrose are:
6,84g× = <em>0,002 moles</em>
<em>B </em>0,72g of ethanol are:
0,72g× = <em>0,0016 moles</em>
<em>C </em>0,40g of sodium hydroxide are:
0,40g× = <em>0,001 moles</em>
Using Van't Hoff factor that describes the number of moles in solution that comes from 1 mole of solute before dissolving, moles in solution for A, B and C are:
A. moles in solution: 0,02 moles×1 = <em>0,02 moles in solution</em>
B. moles in solution: 0,016 moles×1 = <em>0,016 moles in solution</em>
C. moles in solution: 0,01 moles×2 = <em>0,02 moles in solution</em>
As A has the same moles in solution of C, the freezing point depression will be the same in A and C, and the lower freezing point depression will be in B.
Also, the boiling point will be higher in A and C, and then in B.
Thus:
a. C has the lowest freezing point in the group. <em>FALSE. </em>A and C have the lowest feezing point because have the higher freezing point depression.
b. A, B, and C all have different freezing points. <em>FALSE. </em>As moles of A and C are the same, freezing point will be the same for the solutions.
c. A and C have the same boiling point, but B has a lower one. <em>TRUE. </em>
d. The boiling point of C is lower than that of A or B . <em>FALSE. </em>Boiling point of C and A is higher than B.
e. A, B, and C will all have about the same freezing points. <em>FALSE</em>