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susb010prob5 - species Such dilution may also cause an...

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SUSB010 – Problem 5 5. Why does adding water to dilute the juice not change the available acid in the juice? Would it be likely to change the measured pH? Why or why not? The available acid is simply the total number of moles of acid in the solution . It is the sum of the number of moles of acid dissolved in the solution regardless of whether the acid is dissociated or not. This is true for all acids in the solution whose pK a is less than the pK a of the indicator used to determine the end point. We can always choose an indicator with a sufficiently large pK a to be sure we capture a very weak acid. In this exercise, the pK a of phenolphthalein is about 9 so we would not detect any acid with a pK a greater than 9. Adding water to the solution certainly changes the concentration of all dissolved
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Unformatted text preview: species. Such dilution may also cause an increase in the extent of dissociation of the weak acids. But, unless we also add pure acid, the total number of moles of acid will be unchanged. So the total available acid will be unchanged by simply adding more solvent. The same cannot be said of the measured pH. The pH of a solution of a weak acid is a function of its concentration. Consider the simplest case of a single monoprotic weak acid: HA → H + + A- K a = [ H+ ] [ A- ] / [ HA ] n – x x x which for large K a gives us [ H + ] ≈ ( K a c ) 1/2 or pH ≈- ½ log ( K a c ) The pH depends on the log of the molar concentration of the acid!...
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