-58. Hydrofluoric acid is an important reagent used to remove sur- face oxides f

Question

-58. Hydrofluoric acid is an important reagent used to remove sur- face oxides from silicon during the production of semiconduc- tors and computer chips. A chemist working in the semiconductor industry wishes to determine the concentration of an HF solution by using an acid-base titration. A 25.00 mL sample thought to contain approximately 5.00 × 10-3 M HF is to be titrated with a 7.50 × 10-3 M of NaOH. What is the expected pH that will be obtained during this titration for the original sample, after 50% of the HF has been titrated and at equivalence point of this titration? What will be the phH after 30.00 mL of titrant has been added to the HF solution? the

Explanation / Answer

pKa of HF = 3.20

1) 50 % HF titrated .

here it is half equivalence point. here pH = pKa

pH = 3.20

2) at equivalence point volume of NaOH needed = 16.7 mL

here only salt remains

salt concentration = millimoles / total volume

                           = 25 x 5 x 10^-3 / (25 + 16.7)

                         = 3 x 10^-3 M

pH = 7 + 1/2 [pKa + log C]

pH = 7 + 1/2 [3.20 + log 3 x 10^-3]

pH = 7.34

3) 30 mL of base

NaOH milllimoles = 30 x 7.5 x 10^-3 = 0.225

millimoles of HF = 25 x 5 x 10^-3   = 0.125

HF    + NaOH --------------------------> NaF   + H2O

0.125     0.225                                   0             0

0              0.10

strong base remains here

base concentration =0.1 / (25+30) = 1.82 x 10^-3 M

pOH = -log (1.82 x 10^-3)

pOH = 2.74

pH + pOH = 14

pH =11.26

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