Q3. Which test from this experiment tells ethanal from benzaldehyde, and what do you see?
Fehling’s test: ethanal gives a brick red precipitate of copper(I) oxide, while benzaldehyde, an aromatic aldehyde, does not react (Lab Manual, page 94). Both give Tollens’ silver mirror, so that test does not separate them (reasoned).
Q4. How is Tollens’ reagent made in the laboratory?
To about 1 mL of freshly prepared, roughly 2% silver nitrate solution add 1–2 drops of NaOH solution: dark brown silver oxide precipitates. Then add ammonium hydroxide solution drop by drop until the precipitate just dissolves (Lab Manual, page 95).
Q5. Which ion is reduced in Fehling’s and Benedict’s tests, and to what?
Cu2+, held in solution as a tartrate (Fehling’s) or citrate (Benedict’s) complex, is reduced to Cu+, which separates as red copper(I) oxide, Cu2O (Lab Manual, pages 93–95).
Q6. Taking “about 2%” silver nitrate as 2 g in 100 mL, how many moles of Ag+ are in 1 mL, and how many moles of ethanal can those silver ions oxidise at most? (Ag = 108, N = 14, O = 16.)
1 mL holds 0.020 g of AgNO3, M = 108 + 14 + 48 = 170 g mol−1: 0.020/170 = 1.2 × 10−4 mol of Ag+. Each mole of ethanal is oxidised while reducing 2 mol of Ag+, so at most 5.9 × 10−5 mol of aldehyde (reasoned from the Lab Manual, pages 93 and 95; the reading of “2%” is an assumption).