Pre-lab Assignment

• Review pages 204 and 205.
• Define TITRATE, CONCENTRATION, & MOLARITY

In this lab you will be performing a titration to determine the concentration of a base solution.  This lab will be very similar to the last titration you performed, except that the amount of acid will be known and you will find the concentration of the base.  Since you will be able to measure the amount of acid very accurately, you will end up knowing the concentration of base very accurately.  Since this measured concentration of the base may not be exactly the same as the label on the bottle, this is known as "standardizing a base."

The acid to be used is called potassium acid phthalate (or potassium hydrogen phthalate).  This is abbreviated "KHP."  The "H" part is what makes it an acid.  It has a formula mass of 204.23 g/mol.  This acid is a solid, so you will be measuring it by it's mass.

You will use Phenolphthalein ("FEEN-ul-thay-leen") as your indicator again.  Remember that this indicator is clear in acid solution and turns pink in base solution.  An acid solution with phenolphthalein in it will be clear.  If base is added a drop at a time, a pink tint will linger longer and longer after each drop, until it finally stays pink starting exactly when the acid is all neutralized.

Finally, the results section contains a category labeled “mmoles.”  This is a millimole, or a thousandth of a mole.  If a solution is 1.0 M, then it contains one mole in one liter, or one millimole in one mL.  As far as units are concerned, [mol/L]x[mL] = mmol.  Also, [mmole]x[g/mol] = mg.

Procedure

1. Clean and dry all of your glassware, and assemble it as demonstrated.  Put on your goggles.
2. From the dispenser in the front of the room, fill your buret up to the 0.00 mL mark with the sodium hydroxide solution (note that a buret measures amounts of liquid dispensed).  You may need to use the funnel to avoid spillage.  Don't overfill the buret.
3. Record the volume of base in the buret to the proper number of sig figs.  Record it exactly as you read it.  Also record the exact concentration of the NaOH solution.
4. Determine the mass of acid needed to have 0.5 millimoles.  Check this calculatoin with your teacher, then dispense that amount of acid into a beaker.  Dissolve this acid in distilled water (about 40 mL) and add a few drops of indicator.
5. Add base drop-wise to the acid in the beaker, swirling after each drop.  When the solution becomes permanently tinted, record the final volume of base in the buret.  Many people find it easier to see the color change if they keep a sheet of CLEAN white paper under the beaker they are swirling.
6. Repeat for a total of three trials.

Data & Results
 

1.	Write the balanced equation for the reaction of sodium hydroxide with hydrochloric acid.  Include the correct products.
2.	Data Trial #1 Trial #2 Trial #3 mass of weighing dish __________ __________ __________ g mass of dish + KHP __________ __________ __________ g Vi of NaOH __________ __________ __________ mL Vf of NaOH __________ __________ __________ mL
3.	Results Trial #1 Trial #2 Trial #3 mass of KHP used __________ __________ __________ g volume of NaOH used __________ __________ __________ mL mmol KHP used __________ __________ __________ mmol mmol NaOH used __________ __________ __________ mmol [NaOH] __________ __________ __________ M
4.	Calculate the average concentration of the base.  Part of your grade will be based on how accurate AND precise your results are.  [Attach all sample calculations for Trial #1!]

REMINDER - show ALL calculations for trial #1 on an attached sheet

[Standardization of a Base score sheet][MHS Chem page]