1 a What is the pH of a solution that is 0 25 M in HF and 0 15 M F1 The pKa of HF is 3 14 CH 201 Self Study Worksheet 7A Key pH 3 14 log 0 15 M 0 25 M 2 92 base acid so the pH should be less than the pKa b Write out the chemical reaction for when NaOH is added to this HF F1 solution HF OH1 F1 H2O The acid neutralizes all of the OH1 that is added c Upon addition of NaOH to this buffer solution would the pH increase or decrease Increase but only a little Think about what your answers should be d Calculate the pH of the solution after addition of 10 00 mL of 0 050M NaOH to 90 00 mL of this buffer solution H2O Reacts completely OH1 HF 22 5 mmol 0 5 mmol 0 5 22 0 mmol 0 mmol 0 22 M 0 5 F1 13 5 mmol 0 5 14 0 mmol 0 14 M pH 3 14 log 14 0 mmol 22 0 2 94 OR pH 3 14 log 0 14 M 0 24 M 2 94 e Over what pH range is HF F1 a good buffer 2 14 4 14 pKa 1 2 H2PO4 1 has a pKa of 7 21 The purpose of this question is to create a buffer of pH 7 00 using three 2 Phosphate buffers are often used in biochemistry labs The most common buffer system involves H2PO4 HPO4 possible methods a Direct Method How many grams of NaH2PO4 must be added to 500 mL of a 0 400 M HPO4 buffer of pH 7 00 Mm NaH2PO4 is 119 98 g mol 2 to produce a 1 and 7 00 7 21 log 500 ml x 0 400 mmol HPO4 10 0 21 200 mmol HPO4 mmol H2PO4 1 324 4 mmol 2 mmol H2PO4 1 2 mmol H2PO4 1 324 4 mmol NaH2PO4 x 1 mol 1000 mmol x 119 98 g mol 38 9 g 1 solution to produce a buffer of pH 7 00 b Addition of strong base to weak acid How many ml of 3 00 M NaOH must be added to 500 mL of a 0 400 M H2PO4 H2PO4 200 mmol x 200 x mmol 0 mmol 2 H2O Reacts completely 0 x x mmol OH1 HPO4 x x 1 7 00 7 21 log x mmol HPO4 10 0 21 x mmol HPO4 2 200 x mmol H2PO4 2 200 x H2PO4 1 1 123 32 0 6166x x 123 32 1 6166x x 76 28 mmol OH1 76 28 mmol OH1 x ml 3 00 mmol OH1 25 4 mL OH1 CH 201 Self Study Worksheet 7A Key 2 solution to produce a buffer of pH 7 00 c Addition of strong acid to weak base How many ml of 3 00 M HCl must be added to 500 mL of a 0 400 M HPO4 2 HPO4 200 mmol x 200 x mmol 0 mmol 1 H2O Reacts completely 0 x x mmol H3O1 H2PO4 x x 7 00 7 21 log 200 x mmol HPO4 10 0 21 200 x mmol HPO4 2 x mmol H2PO4 2 x H2PO4 1 1 0 6166x 200 x 1 6166x 200 x 123 7 mmol H3O1 123 7 mmol H3O1 x ml 3 00 mmol H3O1 41 2 mL H3O1 3 Consider the titration of 25 00 mL of 0 0800 M NaOH with 0 1000 M HCl a What is the equivalence volume for this titration 25 00 mL OH1 0 0800 M OH1 1 mmol H3O1 1 mmol OH1 0 1000 M H3O1 20 00 mL HCl b What is the pH of the 0 0800 M base solution before any titrant is added NaOH is a strong base so all of it dissociates to OH1 OH1 0 0800 M pOH 1 10 pH 12 90 c What is the pH after 4 00 mL of titrant have been added 4 00 mL 0 1000 M 0 400 mmol H3O1 added 25 00 mL 0 0800 M NaOH 2 00 mmol OH1 originally OH1 H3O1 initial 2 00 mmol 0 40 mmol 0 4 mmol 0 4 mmol final 1 6 mmol 0 mmol 2H2O Strong base in excess OH1 1 6 mmol 25 00 mL 4 00 mL 0 0552 M pOH 1 26 pH 12 74 d What is the pH at the equivalence point 20 00 mL 0 1000 M 2 00 mmol H3O1 added OH1 H3O1 initial 2 00 mmol 2 00 mmol 2 00 mmol 2 00 mmol final 0 mmol 0 mmol 2H2O e What is the pH after 22 00 mL of titrant have been added No strong acid or base in excess pure water pH 7 00 22 00 mL 0 1000 M 2 20 mmol H3O1 added OH1 H3O1 initial 2 00 mmol 2 20 mmol 2 00 mmol 2 00 mmol final 0 0 0 mmol 0 20 mmol 2H2O H3O1 0 200 mmol 25 mL 22 mL 4 26x10 3 pH 2 37 i
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