1. A sample of potassium hydrogen phthalate (KHC 8 H 4 O 4 ), which is an acid,

Question

1. A sample of potassium hydrogen phthalate (KHC8H4O4), which is an acid, with a mass of 0.8473 g is dissolved in water and used to standardize a solution of barium hydroxide. An endpoint is reached when 49.63 mL of barium hydroxide has been added. Determine the molarity (in mol/L) of the barium hydroxide solution. Report your answer to four significant figures.

2. The standardized barium hydroxide solution is then used to titrate 47.88 mL of a solution of phosphoric acid (H3PO4). An endpoint is reached when 33.90 mL of barium hydroxide has been added. Determine the molarity (in mol/L) of the phosphoric acid solution. Report your answer to four significant figures.

Explanation / Answer

1. moles of KHC8H4O4 = grams/molar mass

                                      = 0.8473 g/204.22 g/mol

                                      = 0.00415 mol

KHC8H4O4 is a monoprotic acid

Ba(OH)2 is a diprotic base

So, moles of Ba(OH)2 reacted = 0.00415/2 = 0.002075 mol

Molarity of Ba(OH)2 solution = moles/L of solution

                                              = 0.002075 mol/0.04973 L

                                              = 0.04172 M

2. moles of Ba(OH)2 used = molarity x L of solution

                                          = 0.04172 M x 0.03390 L

                                          = 0.001414 mol

H3PO4 is a triprotic acid

moles of H3PO4 reacted = 0.001414 x 2/3 = 0.000943 mol

molarity of H3PO4 solution = moles/L of solution

                                            = 0.000943 mol/0.04788 L

                                            = 0.01969 M

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