4. 40.0 mL oro This is the equivalence point. Calculate the 0. 100 M barbituric

Question

4. 40.0 mL oro This is the equivalence point. Calculate the 0. 100 M barbituric acid (-9.8 10-) with 40.0 mlL of 0.100 MKOH. Hint 9.3 G e9 40.om OHYo 4.40 5. A sample of 0.3374 g of an unknown monoprotic acid was dissolved in 25.0 ml of water and titrated with 0.1150 M NaOH. The acid required 15.5 mL of base to reach the equivalence point. (a) What is the molar mass of the acid? (b) After 7.25 mL of base had been added in the titration, the pH was found to be 2.85. What is the pK, for the unknown acid?

Explanation / Answer

4. The molar concentration of salt that can be formed = 0.1/2 = 0.05 M

The pH at the equivalence point can be calculated as

pH = 7 + 1/2 (pKa + Log0.05)

Here, pKa = -Log(Ka) = -Log(9.8*10-5) = 4.01

Therefore, pH = 7 + 1/2 (4.01 - 1.30)

i.e. pH = 8.355

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