0.5 gm benzaldehyde and ethanal mixture when heated with Fehling gave 1.43 gm of red precipitate. What is percentage of ethanal in the mixture? [Atomic mass Cu = 63.5, O = 16, C = 12, H = 1]

Only ethanal reacts with Fehling’s solution to form acetic acid and cuprous (red ppt.)

CH₃CHO + 2Cu²⁺ + 2H₂O → CH₃COOH + Cu₂O + 4H⁺
Mole of CH₃CHO = Mole of Cu₂O
$\frac{\mathrm{W}}{44}=\frac{1.43}{143}$ 
Percentage of ethanal $=\frac{1.43}{143}\times \frac{44}{0.5}\times 100=88$