A diprotic weak acid (H_2A) (molecular weight = 125) is used to standardize a KO

Question

A diprotic weak acid (H_2A) (molecular weight = 125) is used to standardize a KOH solution. When 0.400 g of the acid is dissolved in 75 mL of H_2O and then titrated with the KOH, it requires 15.60 mL of the KOH to reach the phenolphthalein endpoint. Calculate the molarity (M) of the KOH solution. Show your calculation method clearly. Answer: _______________ Would the following procedural errors cause the calculated M of KOH in the problem above to be higher or lower than the actual M? Or would the errors not affect the M value? Using complete sentences, explain your reasoning for each question briefly. The weak acid was dissolved in 50 mL of H_2O rather than 75 mL _____________higher __________lower _________not affect When the titration was begun, there was a large air bubble in the buret tip. _____________higher __________lower _________not affect The titration was stopped when a dark pink color was reached instead of stopping at a very light pink. _____________higher __________lower _________not affect

Explanation / Answer

1)

moles of acid = 0.4 / 125 = 3.2 x 10^-3 mol

H2A +   2 KOH   ---------------> K2A   + 2 H2O

Molarity = moles / volume = 3.2 x 10^-3 / 0.075

M1 = 0.0427 M

V1 = 75 mL

n1 = 1

volume of KOH required = V2 = 15.6 mL

M2 = ??

n2 = 2

at end point of titration :

M1 V1 / n1 = M2 V2 / n2

0.0427 x 75 / 1 = M2 x 15.6 / 2

M2 = 0.410 M

Molarity of KOH = 0.410 M

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