Virtual Titration

In this simulation, you will have the chance to review the procedure for stanardizing a solution of NaOH (precisely determining its concentration) by titration with potassium hydrogen phthalate (KHP).

For an actual standardization of NaOH solution with KHP as the promary standard, you will require the following equipment and materials:

Chemicals:
  1. Sodium hydroxide (solid)
  2. Potassium hydrogen phthalate (solid)
  3. deionized water (liquid, oddly enough)
  4. phenolphthalein indicator solution
Glassware:
  1. buret (50 mL)
  2. Elenmeyer flask (125 or 250 mL)
  3. magnetic stirer (optional)
  4. droppers and stirring rods (as needed)

First, a little review.

The process of standardization is the process of determining the precise value of a measured quantity by comparison to a primary standard. In this case, we wish to determine the concentration of a sodium hydroxide (NaOH) solution so that we can use the solution in titratations where precision is required. After the standardization, The NaOH solution will be a secondary standard. We will use potassium hydrogen phthalate (KHP) as a primary standard. KHP is well-suited for this purprose for a very important reasons: KHP is not very hygroscopic, so it doesn't absorb water from the atmosphere. As such, the mass of KHP measured on a balance can be used to determine exactly how many moles of KHP are bing used.

Procedure

  1. Obtain enough NaOH to make approximately 500 mL of 0.2 M NaOH solution. The mass need only be approximate.
  2. Dissolve the NaOH in enough deionized water to make roughly 500 mL of solution. The volume need only be approximate.
  3. Obtain enough KHP so that you will use roughly 20 mL of your NaOH solution (which shoudl have a concentration of approximately 0.2 M.) The mass should be known as precisely as possible (typically to +/- 0.0001 g)
  4. Dissolve the KHP in a small voume of deionized water and add about three drops of phenolphthalein.
  5. Titrate the KHP to the endpoint. Do this carefully as you approach the endpoint so as to get an accurate determination of the actual endpoint!
  6. Repeat the titration two or three more times to get a precise dtermination of the concentration of the NaOH solution concentration.
Why do we use KHP as a primary standard instead of simply measuring the mass of NaOH to determine the number of moles?
Because NaOH is a liquid at room temperature and pressure and it mass is an unrelaibale method for determining the number of moles. Because NaOH is amorphous (has an indeterminate crystal structure) and it mass is an unrelaibale method for determining the number of moles. Because NaOH is hygroscopic (absorbs water from the air) and it mass is an unrelaibale method for determining the number of moles.
KHP has the formula C8H5KO4. What is the molar mass of KHP (to 5 significant figures)?
g/mol
What is the molar mass of NaOH (to 5 significant figures)?
g/mol
You wish to make an NaOH solution that is approximately 0.2 M. What mass of NaOH must you use per L of water to create this solution (to 2 significant digits)?
g
You measure the mass of NaOH(s) using a top loading balance rather than an analytical balance. Why?
Because the mass of NaOH is only needed to be close to our target value. The actual concentration will not be calculated from the mass of NaOH used. Because the mass of NaOH must be known to two decimal places, but not four. Because you are making a mistake! Use the analytical balance for all measurements!
Which piece of glassware shoudl be used to measure the volume of water into which the NaOH is dissolved?
A volumetric flask is to be used in order to insure an exact solution volume. A graduated cylinder is used to keep the volume measurement resonably precise. A beaker is fine. The volume does not need to be perfect. It just needs to be close to the target value.
When you do the standardization titration, you would liek to use a significant enough volume of the titrant so that a small error in a volume rading is only a small fraction of the total value delevered. Let's suppose we are using a 50 mL buret to deliver the NaOH solution. A significant volume might be considered to be about 20 mL. (Less than 10 mL will give us only three significant figures in the volume, for example.) So let's assume we want to use approximately 20 mL of NaOH solution (which we have made to be approximately 0.2 M.)
What mass of KHP should be used so that we use close to 20 mL of NaOH?
g
Suppose you are to perform the standardization. You have prepared the approximately 0.2 M NaOH solution. Now, you measure approximately 0.82 g of KHP. The actual mass you measure is 0.7902 g.
Approximately (to two significan figures) what volume of 0.2 M NaOH solution will you need for the titration of 0.7902 g of KHP?
mL

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