I need help with questions 3, 4, 5 from organic chemistry. Thanks. Question 3 1
ID: 515211 • Letter: I
Question
I need help with questions 3, 4, 5 from organic chemistry. Thanks.
Question 3 1 pts What is the minimum volume of heptane (in mL) required to recrystallize a 6.4 g sample of B? Solubility data for Compound B 13.3 g 100 mL of heptane at 98°C 3.2 g 100 mL heptane at 5°C Please carry maximum number of significant figures through to final answer then round to a whole number. Do not include units D Question 4 1 pts Assuming a student used 80 mL of heptane in question 3 what is the maximum percent recovery of B given that the solution was cooled to 5C? Please carry maximum number of significant figures through to final answer then round to a whole number. Do not include unitsExplanation / Answer
Answer to Question 3)
Recrystallisation is a process of purification of a substance. The process involves making a saturated solution of the substance i.e. dissolving entire stuff in minimum amounts of solvent followed by cooling it to crystallise it again. Thus the solubility of substance at higher temperature determines the quantity of solvent required for making saturated solutions and have efficient recrystallisation.
At 98 deg C
13.3 g of B is soluble in 100 mL heptane
6.4 g of B is soluble in x mL heptane
x = 6.4 g X 100 mL / 13.3 g
= 48.1203 mL
= 49 mL (rounded off to upper number for if you use 48 mL some amount of B will be left dissolved)
Answer to Question 4)
At 5 deg C
3.2 g of B is soluble in 100 mL heptane
y g of B is soluble in 80 mL heptane
y = 3.2 g X 80 mL / 100 mL
= 2.56 g
Therefore, 2.56 g of B will remain in the solution.
The amount of B recrystallized on cooling = Initial amount of B – amount of B remaining in the solution.
= 6.4 -2.56 = 3.84 g
Therefore, 3.84 g of B will be obtained after recrystallization
Maximum % recovery = amount of B remaining in the solution X 100 / Initial amount of B
= 3.84 X 100 / 6.4
= 60 %
Answer to Question 5)
For a substance to remain in the solution and not crystallise out, the solubility of the substance at lower temperature plays the key role. The solubility of the substance at higher temperature is important only to obtain saturation and ensure all of the stuff dissolves.
At 5 deg C
3.2 g of B is soluble in 100 mL heptane
0.96 g of B is soluble in z mL heptane
z = 0.96 g X 100 mL / 3.2 g
= 30 mL
Since the solubility of a compound is lower in a solvent at lower temperatures than at higher temperature, we can confidently say 0.96 g of B is soluble in 30 mL heptane at 98 deg C
Therefore, minimum 30 mL of heptane is required for B to stay in solution after cooling.