Question #92685

What is the pH of a mixture of 5 ml of 0.1 mol/litre sodium acetate and 4 ml of 0.1 mol/litre acetic acid? (pKa of acetic acid at 25oC = 4.76). How is the pH changed on adding 1 ml of 0.1 mol/litre HCl to the above mixture? (Show your answer by means of calculation)

Expert's answer

HCl→H^{+} + Cl^{-}

"\\frac {1L}{1000mL} \\cdot \\frac {0.1 mol}{1L} \\cdot 1 mL = 0.0001 \\space moles \\space [H+]"

moles of CH_{3}COOH = M*V = 0.1 mol/L * 0.004 L = 0.0004 moles

moles of CH_{3}COO^{-} = moles of CH_{3}COONa = 0.1 mol/L * 0.005 L = 0.0005 moles

pK_{a}=4.76

K_{a}=1.74*10^{-5}

"Ka = \\frac {[H^+] [CH_3COO^-]} {CH_3COOH]}"

"[H^+] = \\frac {1.74 \\cdot 10^ {-5} \\cdot 0.0005}{0.0004} = 2.175 \\cdot 10^ {-5}"

pH=-log_{10}(2.175*10^{-5}) = 4.66

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