A mixture of 0.535g of ethanol and acetaldehyde when treated with a Fehling solution gave 1.2g of a red precipitate.

Question

A mixture of $0.535 g$ of ethanol and acetaldehyde when treated with a Fehling solution gave $1 \cdot 2 g$ of a red precipitate. What is the percentage of acetaldehyde in the mixture ? $( Cu =63 \cdot 5)$

Answer 1

SOLUTION : On the basis of reactions studied in organic chemistry we know that an aldehyde on bcing warmed with Fehling solution yields a red precipitate of $Cu _{2} O$, and it is itself oxidised. In this problem

$CH _{3} CHO +2 CuO \rightarrow CH _{3} COOH + Cu _{2} O$

(Acctaldehyde) (Acetic acid) Red ppt.

Mol. wt. of $CH _{3} CHO =12+3+12+1+16=44 g / mole$

Mol. wt. of $Cu _{2} O =63.5 \times 2+16=143 g / mole$

$\because 143 g$ of $Cu _{2} O$ ohtained by $44 g$ of acetaldehyde

$\therefore 1 \cdot 2 g$ of $Cu _{2} O$ úbtained by $\frac{44 \times 1 \cdot 2}{143} g =0 \cdot 369 g$ acetaldehyde.

The percentage of acctaldchyde in the mixture $=\frac{0 \cdot 369}{0 \cdot 535} \times 100=68.97 \%$.