exam 1

Question 1

difference between gravity and vacuum filtration

Answer 1

gravity is done when you want to keep the liquid and vacuum is done when you want to keep the solid.

vacuum is preferred because it takes the maximum amount of amount of solvent from surface of crystal

Question 2

if you add more solvent what does it to the yield and purity of a crystal

Answer 2

you lose more yield but it will be more pure

Question 3

best solvent pairs for recrystallization

Answer 3

one solvent is polar and the other is non polar, one solvent should be soluble at the boiling point of the compound while the other is insoluble at high temperatures.

Question 4

3 isomers of c6H14 with all different melting points, whats true about them?

Answer 4

The polarity is approximately the same

the surface area decreases when it gets smaller

more symmetrical it is the more the London dispersion forces decrease

more surface area the higher the boiling point

Question 5

when a solid is dissolved in a solvent which of the following procedures leads to crystals

Answer 5

cooling the liquid
partial evaporation of the solvent
addition of a seed crystal
addition of a solvent in which compound has low solubility

Question 6

will there be better purity of the product of recrystallization if the solution containing solid is cooled slowly?

Answer 6

yes, less likely that impurities will be entrapped in the growing crystal

Question 7

in recrystallization which of the following steps lead to a maximized yield of product?

Answer 7

minimal amount of solvent

Question 8

if you heat isopropyl up in a water bath what will the temp be of the isopropyl acetate inside the flask and the gas phase above the liquid inside the flask?

Answer 8

temp of isopropyl acetate can only heat up to its own boiling point of 88 C while the gas phase has to be higher and hotter than that because it can only get up to 88 in liquid form

Question 9

in the distillation apparatus a thermometer positioned too high influence of vapor pressure

Answer 9

thermometer that is too high will have the vapors not enveloping the thermometer bulb completely and the bulb is only partially heated getting a reading of a low temp

Question 10

during fractional distillation after about 50% of liquid has been distilled from the pot into receiver flasks, the temp shown by thermometer drops, why?

Answer 10

there is barely any of the vapor of the boiling liquid left, while the vapors of the higher boiling liquid have not reached the top of the column. There is not enough vapor to heat the thermometer

Question 11

why does stirring prevent “bumping” when a strongly heated liquid is at it’s boiling point?

Answer 11

the stirring prevents bumping by keeping the heat distribution maintained it doesn’t boil over and it helps the liquid boil more smoothly by keeping the bubbles controlled. More vigorous it is stirred the less chance of bumping

Question 12

distillation under vacuum (reduced atmospheric pressure) relative to a distillation at one atmosphere, what is true?

Answer 12

the boiling points of non polar components in the mixture decreases

Question 13

ensure reproducibility of Rf values is maximized?

Answer 13

atmosphere in TLC chamber, chambers should be closed so solvent in chamber can saturate the atmosphere so molecules are in contact with the silica gel surface

Temp of TLC’s should be developed at 25 C

Question 14

how can using a pen alter the TLC plate

Answer 14

pens have organic molecules and additional spots may appear

Question 15

the solvent is higher up than the origin for the TLC plate

Answer 15

origin should be higher or else the sample will dissolve into the solvent reservoir and no spots or streaky spots will be observed

Question 16

applying to little of a sample to TLC plate

Answer 16

hard to get accurate Rf value and might not enough spots produced

Question 17

Rf is the inverse of

Answer 17

polarity

so molecules with increasing polarity will have opposite order of increasing Rf

Question 18

On a TLC plate if one drop of water is at one end and on the other is hexane what will happen

Answer 18

polar and non polar

water won’t spread out but hexane (non polar) will

if they interact they won’t mix well and the nonpolar will go over the polar

Question 19

are hydrophobic molecules attracted to water?

Answer 19

yes because there is a slight dipole charge, but they don’t go well together

Question 20

what is true of hydrophobic/non polar liquids

Answer 20

hydrophobic liquids like hexane spread out and form a thin film on polar surfaces (like glass)

hydrophobic liquids rise in capillary (undergo capillary action)

Question 21

when separating three compounds of medium but different polarity by TLC which of the following factors will make separation occur?

Answer 21

choosing a solvent of medium polarity

creating very small spots at the origin when applying the compound to the plate

Question 22

what is fluorescence quenching in TLC

Answer 22

it is a technique in which the presence of a compound on a plate is detected by the absence of fluorescence

spots will appear dark on bright background

Question 23

pka of HI

Answer 23

-10

Question 24

pka of H2SO4 and HCL

Answer 24

-5

Question 25

pka of oxinium ions (protonated water, alcohol, and ethers)

Answer 25

0

Question 26

pka of carboxylic acids

Answer 26

5

Question 27

pka of phenols, protonated amines, and thiols

Answer 27

10

Question 28

the pka of water and alcohol

Answer 28

15

Question 29

pka of amines

Answer 29

35 (uncharged n with h attached)

Question 30

acidic hydrogen atom in compound A will undergo a proton exchange reaction if

Answer 30

the newly formed bond is less acidc than the bond that was broken in compound A

the proton finds an atom to which it can form a more stable bond

Question 31

to dry an organic solution and 2 common reagents

Answer 31

means to remove water in our organic solvent layer

reagents = sodium sulfate and magnesium sulfate

you can stop adding the drying agent when it doesnt clump anymore

Question 32

2 immiscible liquids one is concave one is convex

Answer 32

adhesive and cohesive forces, when the adhesive is stronger than the adhesive then the liquid is concave. When cohesive is stronger than adhesive it will be convex

Question 33

aqueous HCL

Answer 33

CL + H3O+

Question 34

when a molecule is hydrophilic and hydrophobic like soap

Answer 34

can interact with grease molecules and form around them called micelles in the hydrophobic area

but when interacted with water it will be emulsified and the grease and oil will be washed away