In the experiment, we used titration to determine the total acid content of samples, which we reported in terms of molarity. Reporting acidity through pH measurements is quite different, in that we can only measure the amount of the acid in its ionized form. You were tasked to investigate a clear aqueous solution of an unknown monoprotic acid. You decided to use two methods to gather data. Method 1 – TITRATION: A 10. mL aliquot of the sample was diluted with 25 mL distilled water. Two drops of phenolphthalein were added and then it was titrated 3.54 mL of 0.048 M standardized NaOH to the endpoint. Method 2 – pH STRIP: You took 1 mL of the sample and used a pH strip to estimate the pH, which turned out to be around 3.3. Another 1 mL of the sample was diluted with 9 mL of water. The pH was taken again and is now around 3.8. a) Calculate the molarity of the acid using the titration data. b) If we assume that the titrated unknown is a strong acid, predict the pH of the sample. c) Using pH strip results, calculate the [H+ ] concentration in the sample before the dilution. d) Comparing the results from the two methods, deduce if the unknown is really a strong acid, or if it’s a weak acid. Defend in one sentence. e) Rationalize the two pH readings in the pH strip test before and after dilution. Is this to be expected?
In the experiment, we used titration to determine the total acid content of
samples, which we reported in terms of molarity. Reporting acidity through pH
measurements is quite different, in that we can only measure the amount of the
acid in its ionized form. You were tasked to investigate a clear aqueous solution
of an unknown monoprotic acid. You decided to use two methods to
gather data.
Method 1 – TITRATION: A 10. mL aliquot of the sample was diluted with 25 mL
distilled water. Two drops of phenolphthalein were added and then it was titrated
3.54 mL of 0.048 M standardized NaOH to the endpoint.
Method 2 – pH STRIP: You took 1 mL of the sample and used a pH strip to
estimate the pH, which turned out to be around 3.3. Another 1 mL of the sample
was diluted with 9 mL of water. The pH was taken again and is now around 3.8.
a) Calculate the molarity of the acid using the titration data.
b) If we assume that the titrated unknown is a strong acid, predict the
pH of the sample.
c) Using pH strip results, calculate the [H+
] concentration in the
sample before the dilution.
d) Comparing the results from the two methods, deduce if the
unknown is really a strong acid, or if it’s a weak acid. Defend in one
sentence.
e) Rationalize the two pH readings in the pH strip test before and after
dilution. Is this to be expected?
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