Question

21. Two of your friends, Rex and Cody, are having a disagreement. Rex argues that 0.020 M HCl is more acidic than 5.0 M CH3CH2COOH (pKa = 4.87) because nitric acid is a strong acid. Cody argues, however, that 5.0 M CH3CH2COOH (pKa = 4.87) is more acidic than 0.020 M HNO3 because it has a higher concentration. Who is correct? Prove your point by calculating both pHs.

Answer 1

Solution

$CH_3CH_2COOH(pKa=4.87)$

pKa = 4.87

logka= -4.87

ka = $1.34\times10^{-5}$

$[H^+]=\sqrt{1.34\times10^{-5}\times5.0}$

$=8.19\times10^{-3}$

pH= -log(8.19$\times10^{-3})$ = 2.09

pH of HCl$=log(0.020)=1.70$

pH of HN0₃$=log(0.020)=1.70$

Rex is correct propanoic acid is weaker compared to nitric acid because the lower the value of pH the stronger the acid.