a.k.a. What to do when things dont work out quite the way you
thought they would.
Slide 3
'Twas the night to make crystals, and all through the hood,
Compounds were reacting; As I'd hoped that they would. The hood
door I'd closed with the greatest of care, To keep noxious vapors
from fouling the air. The reflux condensor was hooked to the tap,
And the high vacuum pump had a freshly filled trap. I patiently
waited to finish my task, While boiling chips merrily danced in the
flask. Then from the pump there arose such a clatter, That I sprang
from my chair to see what was the matter. Away to the fume hood! Up
with the door! And half my product foamed out on the floor.
Slide 4
Then what to my watering eyes should appear, But a viscous
black oil which had once been so clear. I turned the pump off in a
terrible rush, And the oil that sucked back filled the line up with
mush. The ether boiled out of the flask with a splash, And hitting
the mantle went up with a flash. My nose turned quite ruddy, my
eyebrows went bare, The blast had singed off nearly half of my
hair. I shut the hood door with a violent wrench, As acid burned
holes in the floor and the bench. I flushed it with water, and to
my dismay, Found sodium hydride had spilled in to the fray.
Slide 5
And then the fire got way out of hand, I managed to quench it
with buckets of sand. With aqueous base I diluted the crud, Then
shoveled up seven big buckets of mud. I extracted the slurry again
and again With ether and then with dichloromethane. Chromatographic
techniques were applied Several times 'til the product had been
purified. I finally viewed with a satisfied smile, One half a gram
in a shiny new vial. I mailed the yield report to my boss, Ninety
percent (allowing for loss). "Good work," said the boss in the
answering mail, "Use same condition on a preparative scale."
Slide 6
Time Concentration N 2 (g) + 3 H 2 (g) 2 NH 3 (g) H2H2 NH 3
N2N2 Percent Yield 2 L 4 L 2 L = 2/4 x 100 = 50% 6 L
Slide 7
Example: What is the percent yield of the reaction below if
2.5g of hydrogen gas only produces 18.0 g of water? 2 H 2 + O 2 2 H
2 O Step 1: Calculate the gfm of H 2 and H 2 O and write them under
the grams line in your stoichiometry chart. 2.016 g/mol18.015 g/mol
Step 2: Calculate how many grams of water would be formed from the
2.5 g of hydrogen. 2.5 g Step 3: Divide what was obtained
experimentally (18.0 g H 2 O) by the theoretical yield of water and
multiply by 100. 22.4 g Theoretical yield g: gfm: mol:1.24 mol
x