Question

Students performing vinegar analyses made several mistakes in their experiments. Explain in detail how each error below will affect the final result (% weight of acetic acid in vinegar) of each student's experiment. Show all steps of logic followed in analyzing the effects of the mistake (remember that the experiment had two parts):

1.Student A used a wet beaker to transfer the NaOH solution to be standardized from its original container to the buret.

2.Student B did not fill the tip of the buret with titrant before starting the NaOH standardization, so the tip contained air when the titration was started, but was filled after the first standardization.

3.Student C over-titrated the php solution, but ignored it and continued on with calculations.

4.Student D was in a hurry to leave the lab. He finished the titration of vinegar without waiting for the pink color to persist for 15 seconds. He recorded the volume of NaOH used, just to notice that the pink color disappeared. He ignored this observation and proceeded with calculations.

Answer 1

We should understand the following facts before addressing or commenting on the mistakes done by the students:

The exact concentration of the titrant, i.e. NaOH should be known, in Molarity / Normality. This is required to calculate the weight of acetic acid (HAc) present in the vinegar. The reaction takes place as follows:

HAc (aq.) + NaOH (aq.) ® NaAc (aq.) + H2O (l)

From the above balanced reaction (on mole basis), if we know the concentration of the NaOH [how much weight (g) is dissolved in how much of water (L)], the amount of acetic acid (HAc) in g can be calculated.

The standardization of NaOH against the primary standard PHP (potassium hydrogen phthalate), actually gives the exact amount of NaOH present in the titrant (sodium hydroxide solution).

KHC8H4O4 (aq.) + NaOH (aq.) ® KNaC8H4O4 (aq.) + H2O (l)

Now, we can comment on their mistakes as follows:

1. Student A used a wet beaker to transfer the NaOH solution to be standardized from its original container to the burette.

In this case the wet beaker is having some water, hence the volume of NaOH taken in it, will be slightly diluted, and after standardization he will get a value which will be slightly lower than his stock solution (in original container). But he will use this value and assign the strength of NaOH present in the container, which is actually higher than the standardized solution. Now when he will titrate the vinegar, he will use the lower strength value of the NaOH in his calculations get the lower value of the acetic acid in vinegar.

2. Student B did not fill the tip of the buret with titrant before starting the NaOH standardization, so the tip contained air when the titration was started, but was filled after the first standardization.

In this case the actual volume of the NaOH will be very negligibly less, and in the second standardization, it is right volume is dispensed for neutralization of the PHP. The average of two standardizations will be very close to the actual one. When he determines the acetic acid content in the vinegar, he will get the values very near to the actual one, and will be within the analytical variations.

3. Student C over-titrated the php solution, but ignored it and continued on with calculations.

Over titration means he got the higher value of NaOH concentration and again will wrongly label the container with higher conc. value. Now when he will titrate the vinegar, he will report the higher percentage of the acetic acid, as he labelled hos NaOH with higher conc. than the actual one.

4. Student D was in a hurry to leave the lab. He finished the titration of vinegar without waiting for the pink color to persist for 15 seconds. He recorded the volume of NaOH used, just to notice that the pink color disappeared. He ignored this observation and proceeded with calculations.

In this case, he did not standardize the NaOH solution as mentioned in the lab observation. Hence, the actual conc. of his NaOH is not known. Secondly he did not continued the reaction till the end point (permanent pink color). Hence, when he is calculating the conc. of acetic acid, he very likely to get either higher or lower results, but certainly not the actual one.