Potassium acid phthalate, KHP (KHCsHO4), is a primary standard reagent used to d

Question

Potassium acid phthalate, KHP (KHCsHO4), is a primary standard reagent used to detern exactly the concentration of a solution of base, such as NaOH, whose approximate concentration is known. KHP is a weak acid, by NaOH is and the equation for the neutralization of KHP COOK + H2O + NaOH COONa COOH KHP Potassium acid Phthalate 1. Zack dissolved 0.28 grams of KHP in 15.0 mL of water. He found out that it required 18.5 mL of NaOH to reach the endpoint of the titration. a. How many moles of the acid, KHP, were originally present before the addition of the base? mal KHP b. Calculate the concentration of the NaOH solution? 3.94 10-3ol KHP 0.213

Explanation / Answer

At end point . salt remians.

HP-   +   OH-    --------------> P2- +   H2O

At end point : moles of acid = moles of base

salt concentration = millimoles = total volume

                              = 3.94 / 15 + 18.5

                              = 0.1176 M

Ka = 3.89 x 10^-6

Kb = 2.57 x 10^-9

P2-    +   H2O    ------------------> PH-   +   OH-

0.1176                                          0             0

0.1176 - x                                      x             x

Kb = x^2 / 0.1176 - x

2.57 x 10^-9 = x^2 / 0.1176 - x

x = 1.74 x 10^-5

[OH-] = 1.74 x 10^-5 M

pOH = -log (1.74 x 10^-5 )

pOH = 4.76

pH = 9.24

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