Need help on part B, my answer (5.25) was incorrect. A titration involves adding

Question

Need help on part B, my answer (5.25) was incorrect.

A titration involves adding a reactant of known quantity to a solution of an another reactant while monitoring the equilibrium concentrations. This allows one to determine the concentration of the second reactant. The equation for the reaction of a generic weak acid HA with a strong base is HA(aq) + OH-(aq) rightarrow A-(aq) + H3O+(aq) A certain weak acid, HA, with a Ka value of 5.61 Times 10-6, is titrated with NaOH. A solution is made by titrating 7.00mmol (millimoles) of HA and 1.00mmol of the strong base. What is the resulting pH? Express the pH numerically to two decimal places. More strong base is added until the equivalence point is reached. What is the pH of this solution at the equivalence point if the total volume is 37.0mL ? Express the pH numerically to two decimal places.

Explanation / Answer

Part A : weak acid + strong base acts as buffer solution.

pH of acidic buffer = pKa + log [base/acid]

pKa = -log 5.61*10^-6

    = 5.25

pH = 5.25 + log (1/7-1)

   = 5.25 + log (1/6)

   = 4.47

Part B

at equivalence point the pH of solution depeds on only strong base added.

No of mol of acid = No of mol of base = 7 mmol

pH = 7+1/2[pKa+log C]

C = 7/37 = 0.189 M

pH = 7 + 1/2[5.25 + log 0.189]

   = 9.26

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