Lab exam

Question 1

Precipitate

Answer 1

lower purity than crystals

Question 2

Crystallization

Answer 2

purification technique for solids: impure solid dissolved in hot solvent, and hot solution slowly cool. Developing crystals are high purity. Solid should be very soluble in hot solvent and minimally soluble in cold solvent.

Question 3

Steps for crystallization

Answer 3

1. fully dissolve in hot solvent
2. upon cooling- soluble impurity will remain dissolved in mother liquor while desired compound crystallizes

Question 4

Hot filtration steps

Answer 4

use fluted filter paper, short stem funnel, clean 125 mL Erlenmeyer flask
Impurity embedded in crystalline matrix is insoluble in crystallization solvent. Impure solid is heated in hot solvent needed to dissolve. Insoluble material is filtered while solution is kept hot and desired compound crystallized.

Question 5

Crystallization solvent

Answer 5

solvent should have <100 degrees boiling point
Room temp dissolves NO
Room temp stay solid YES
Boiling stay solid NO
Boiling dissolves YES
Room temp dissolves NO
Room temp stays solid YES

Question 6

What can a solid’s melting point be affected by?

Answer 6

impurities lower and broaden melting point range

Question 7

How to maximize the amount of crystals?

Answer 7

Use MINIMUM amount of hot solvent
Increase purity by cooling SLOWLY
Increase recovery by cooling in an ice bath
Use solids of moderate solubility at low temps: lessens quantity of compound lost to mother liquor

Question 8

Hot versus Regular Filtration

Answer 8

Hot (collects impurities): solution filtered while still hot and compound of interest dissolves; insoluble impurities left on filter paper

Regular (collects product): if all impurities are soluble then recrystallized product can be filtered

Question 9

What is distillation?

Answer 9

separates 2 or more liquids based on boiling point (lower boiling point boils off first)

Question 10

What is Raoult’s and Dalton’s Laws?

Answer 10

PA=PoAXA
Ptotal=PA + PB

Question 11

Composition of Distillate is?

Answer 11

equal to the composition of vapors

Question 12

As temp increases…

Answer 12

a greater % of molecules have sufficient energy to overcome IMFs holding them in liquid phase. Vapor pressure ALWAYS increases with temperature. A compound with lower boiling point has a higher vapor pressure than one with higher boiling point

Question 13

What is simple distillation?

Answer 13

separate liquid from non-volatile impurities and the difference between boiling points is >100 degrees

Question 14

What is fractional distillation (what does fractioning column and condensor do)?

Answer 14

when bp difference is <100 degrees, the fractioning column (between three way adaptor and heating mantle) allows for many successive distillations to take place (indentations with lots of surface area). The condensor (between three way adaptor and vaccum adaptor) allows for cold water to circulate through apparatus.

Question 15

Where is the water out and water in tubes on the condensor?

Answer 15

water out is farthest left, water in is the right

Question 16

Steps for fractional distillation?

Answer 16

1. Circulate cold water in condensor
2. Increase heat
3. Increase for 1 drop per second once dripping

Question 17

What is vapor pressure?

Answer 17

pressure exerted by a vapor in equilibrium with its condensed liquid at a given temperature in a closed system.

Question 18

When is the boiling point in relation to vapor pressure?

Answer 18

When the vapor pressure is equal to the pressure surrounding the liquid.

Question 19

Distillation curve

Answer 19

Lower curve: boiling point (composition of liquid mixture vs temp)
Upper curve: vaporization concentration (composition of gas vs temp)

Question 20

What are azeotropes?

Answer 20

non-ideal behavior and don’t follow Raoult’s Law