Bring the lecture that lost you or the homework that isn’t working out. We’ll go through it with you, then show you how to approach the next one. Here is how that looks on a few problems, one from each course.
Taught by Gavin Brown and Felix Campbell, chemistry PhD students at the University of Arkansas.
Worked exampleOrganic Chemistry I, substitution
(R)-2-Bromobutane is treated with sodium cyanide in DMSO. Draw the product, including stereochemistry.
Backside attack inverts the stereocenter: (R) goes in, (S) comes out.
Inversion doesn’t always change the letter, so re-rank the groups every time.
One from each course we teach. Printed text is the question; blue is our working.
A stoichiometry problem: turning a mass of fuel into a mass of product.
Request help with high school or APSample problemStoichiometry
How many grams of CO2 form when 22.0 g of propane, C3H8, burns completely?
C3H8 + 5 O2 → 3 CO2 + 4 H2O
22.0 g×1 mol C3H844.10 g×3 mol CO21 mol C3H8×44.01 g1 mol CO2= 65.9 g CO2
Grams → moles → mole ratio → grams. The route is the same every time.
A weak-acid equilibrium: setting up the table, then checking the shortcut.
Request help with general chemistrySample problemWeak acids
What is the pH of 0.10 M acetic acid? (Ka = 1.8 × 10−5)
| CH3COOH | H+ | CH3COO− | |
|---|---|---|---|
| I | 0.10 | 0 | 0 |
| C | −x | +x | +x |
| E | 0.10 − x | x | x |
Ka =x20.10 − x≈x20.10= 1.8 × 10−5
x = √(1.8 × 10−6) = 1.34 × 10−3 M
pH = −log(1.34 × 10−3) = 2.87
Check the shortcut: x is 1.3% of 0.10. Under 5%, so dropping it was fair.
A short synthesis, planned backwards from the product.
Request help with organic chemistrySample problemSynthesis
Propose a synthesis of 2-butanone from trans-2-butene.
Work backwards. A ketone comes from a secondary alcohol, and the alcohol comes from adding water across the alkene. The alkene is symmetrical, so there is only one alcohol to make.
Tell us your course and the topics you’re working on. Your tutor will reply to confirm rates, timing and format.