This experiment is done over a two-week period. There are two separate lab reports, however, and you will receive two separate grades. The first lab report is on the Standardization of NaOH (Page 169 of the lab manual), whereas the second lab report is on the Equivalent Weight of an Unknown (Page 175 of the lab manual; see separate help file). To make things easier, in the discussion to follow, we will do each of these lab reports separately.

Click here for the portion on Equivalent Weight of an Unknown (second week)

Standardization of NaOH

Overview 

During the first week of this experiment, you prepared a sodium hydroxide solution (which was approximately 0.1 M) by diluting a more concentrated NaOH solution. Since NaOH solutions cannot be prepared by mass to be an exact concentration (solid NaOH is too reactive), you then standardized your NaOH solution by titrating weighed samples of a primary standard acidic substance, potassium hydrogen phthalate (KHC₈H₄O₄, "KHP", molar mass 204.2 g). KHP has one acidic hydrogen atom, and reacts with NaOH on a 1:1 stoichiometric basis:

The net ionic equation for this process (as with any acid base reaction in aqueous solution) is

Since all the reactants and products of this neutralization reaction are colorless, a few drops of an indicator (phenolphthalein) were added to each KHP sample to be titrated. Phenolphthalein is colorless in acidic solution, but turns red in basic solution. When one drop more NaOH is added than is required to react with the KHP in a sample, the sample becomes basic and the indicator changes color. We take the point where the indicator color first appears as an indication that we have added NaOH in an equivalent amount to the KHP present in the sample.

Data 

The data recorded on Page 169 consists of two parts: the masses of the KHP samples, and the volumes of NaOH solution used to titrate the samples.

You weighed out your four (or 5) samples by a "new" technique-weighing by difference. In the past, if you had wanted to weigh out a sample, you first weighed an empty container, and then reweighed the container after the sample had been added to it. Obviously, the difference in these two masses is the mass of the sample itself.

In this experiment, you did things differently. You first placed enough KHP for four (or 5) samples into a small beaker and weighed the whole thing (you did not determine the mass of the empty beaker).

Then you removed your first sample and weighed the beaker containing the remainder of the KHP. The difference in mass before and after you removed the first sample is, of course, the mass of the first sample itself.

You then continued removing the remaining samples, weighing the beaker after removing each sample, until you had all your samples measured out. The reason for this method is that it requires fewer weighings. To weigh out four samples by this method, you would only have to make five mass determinations: the mass before any samples were taken, and the masses after each of the four samples had been removed. If you weighed the samples out by the earlier method, you would have needed to make eight mass determinations (mass of empty container, mass of container with sample). By the weighing by difference method, you also did not have to dry out the Erlenmeyer flasks into which you placed the samples.

A. Mass of KHP Samples

B. Buret Readings

Calculations 

Page 170, Part II

The sample calculations in Parts IIA, B, C, and D are for Sample 1 above.

A. Mass of KHP in Sample 1

According to the way the "weighing by difference" technique described in the overview works, the mass of Sample 1 is just the difference in mass of the first two weighings of the beaker containing the KHP.

B. Moles KHP in Sample 1

A common mistake students make in this experiment is to confuse the abbreviation "KHP" with a true chemical formula when calculating the molar mass. The "P" in "KHP" is an abbreviation for the phthalate ion (C₈H₄O^-) not a phosphorus atom: the molar mass of KHP is 204.2 g.

C. Volume of NaOH in liters for Sample 1

This is not very hard! We measured the volume of NaOH using a buret which was calibrated in milliliters, but we want to calculate the Molarity of the NaOH solution (see Part D below) in liters. The volume of NaOH, in milliliters, used to titrate Sample 1 is just the difference between the buret readings.

By this point, you may realize that to change a volume from mL to L just means moving the decimal point three places to the left. But, in terms of a formal calculation

D. Molarity of NaOH from Sample 1

Sample 1 contained 0.002509 mole of KHP, and required 0.02275 L of NaOH to reach the endpoint (indicator color change). Since the reaction between KHP and NaOH is of 1:1 stoichiometry, this means that 0.002509 mole of NaOH must have been used. Therefore, the molarity of the NaOH solution based on the Sample 1 Titration is

Always ask yourself if your answers are consistent. The instructions for preparing the NaOH solution were supposed to lead to a solution that was approximately 0.1 M. Our precise molarity of 0.1103 M is consistent with this.

Results 

Part III, Page 170-171

The table at the beginning of this section just summarizes your calculations of Molarity for the other KHP samples you titrated. Remember that the mass of each successive sample is calculated by subtracting the masses of the beaker of KHP before/after the sample was taken.

If you weighed your samples carefully, and titrated them equally carefully, your four values for the molarity will have a very small spread among the values. Since you had only one NaOH solution, it has only one true molarity. Your grade on this part of the experiment is based on the precision of your titrations (how close the molarities are to each other).

If you need review on calculation of averages, deviations, and percent deviations, please look at the earlier titration experiments you performed.