Gen Chem
What do each of the four NFPA diamond regions blue, red, yellow, and white represent?
Blue health; red flammability; yellow instability/reactivity; white special hazards.
True or False. The first temperature recorded is when the sample is completely liquid.
False: first number is first obvious liquid; second is complete liquefaction.
How is the minimum solvent volume determined, and why is it important?
Add hot solvent in small portions until just dissolved; minimum maximizes crystallization and recovery.
How can aromatic C–H out-of-plane bending distinguish mono-, meta-, and para-disubstituted rings?
Their diagnostic out-of-plane bands/ranges differ in the provided table.
Where should the thermometer bulb be placed in a distillation apparatus, and why?
Bulb level with side arm so it measures vapor entering condenser.
How many mL of 6.00 M stock make 250.0 mL of 0.150 M solution?
0.200 M
A student reports a melting point as 148 °C. What is wrong?
It must be a range with units, e.g., 147–149 °C; one value omits the observed start and finish.
T or F? The recrystallization solvent should have a boiling point above the melting point of the solid being dissolved.
False; boiling point of solvent must be below melting point of solid (to prevent solid from melting as opposed to dissolving)
How would IR distinguish an alcohol O–H from a carboxylic-acid O–H?
Alcohol O–H broad 3300–3500; acid O–H very broad 2500–3300.
Compare simple and fractional distillation by vaporization-condensation cycles and successful-use conditions.
Simple=one cycle, good for large BP difference/nonvolatile impurity; fractional=many cycles, better for close BPs
I calculated theoretical yield is 5.00 g but I actually got is 4.20 g pure compound. Calculate percent yield.
84.0%.
Explain why a pure compound usually has a sharp range while an impure sample generally has a lower and broader range.
Pure crystals have a uniform lattice and narrow range; impurities disrupt the lattice and usually depress and broaden the range.
How does decolorizing charcoal work? Why use only a small amount?
Large surface-area charcoal adsorbs colored impurities; excess also adsorbs desired compound
Assign signals near 9–10, 7, 3–4, and 1 ppm (general)
Aldehyde H; aromatic H; H near O/N/halogen; simple alkyl H.
During simple distillation of 50:50 A/B, why is first distillate richer in A and why does pot temperature rise?
A is more volatile so initial vapor is A-rich; as A leaves, pot becomes B-rich and BP rises.
For 2 H2 + O2 → 2 H2O, 3.00 mol H2 reacts with 1.00 mol O2. Find limiting reagent and theoretical moles H2O.
O2 limiting; 2.00 mol H2O; 3.00 mol H2
U melts at 98.7–101.0 °C. U+A melts at 98.4–100.0 °C; U+B melts at 82.0–88.5 °C. Identify U and explain.
A; U+A stays near and relatively sharp, while U+B is strongly depressed and broadened
What is oiling out? Give the corrective procedure.
Oil formation instead of crystals; add solvent, reheat to dissolve, cool slowly again
Compound A is an aldehyde and B is a ketone. State one IR and one 1H NMR difference
A has aldehyde C–H IR bands and a proton near 9–10 ppm; B lacks both
Define theoretical plate and HETP. How do plates and HETP affect efficiency?
One equilibrium vaporization-condensation cycle; HETP is height per plate. More plates/lower HETP improve separation.
Interpret blue 3, red 2, yellow 1, white OX
Blue 3 serious health; red 2 moderate flammability; yellow 1 normally stable but unstable when heated; white OX oxidizer.
Define melting point and explain what happens to intermolecular forces and the crystal lattice as a solid melts.
Solid-to-liquid phase change; heat disrupts the crystal lattice by overcoming intermolecular attractions, and temperature stays nearly constant during the phase change.
List the properties of an ideal recrystallization solvent.
Desired solid soluble hot but poorly soluble cold; impurities insoluble or always soluble; volatile, inert, preferably safe; BP below solid MP
A spectrum shows five aromatic H, a quartet integrating to two H, and a triplet integrating to three H. Propose a fragment and explain
Likely an ethyl group attached to a monosubstituted benzene: quartet/ triplet are CH2/CH3 and five aromatic H are present.
List four retention-time factors; identify the two easily changed during a run.
BP, column length, carrier flow, oven temperature; flow and oven are easiest to change.