A student measured 20 drops of vinegar and one drop of phenolphthalein solution

Question

A student measured 20 drops of vinegar and one drop of phenolphthalein solution into a well of her well plate. After she had added 35 drops of NaOH solution to the same well, the well was full, but she had still not reached the titration end point. She then took a clean pipet and transferred some of the titration mixture into a second well. She rinsed the pipet with a few drops of distilled water and transferred the rinse water into the titration mixture in the second well. Then she continued adding NaOH solution to both wells until she reached the end point in both wells. She then recorded the total number of drops of NaOH solution added to both wells and used it to calculate the data for her vinegar titration. Briefly comment on her procedure and the accuracy of her results.

Explanation / Answer

The monoprotic- monobasic titration endpoint is reached only when-

            The number of moles of acid (vinegar) = number of moles of base (NaOH)

Dilution reduces concentration BUT it does not affect the number of moles of a solute in a solution. For example, there is 1 mole NaOH in 1 L of 1M NaOH solution. If this solution is diluted to 10 L, the concentration is reduced to 0.1 M. However, the total number of moles of NaOH still remains the same as that of initial solution i.e. 1 mole.

So, the procedure does not affect the result and accuracy as long as she counts the NaOH volume consumed during titration correctly.

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