MHS Chemistry
Simple Titration

Pre-lab Assignment

Purpose and Theory

In this lab you will be performing a titration to determine the concentration of an acid solution.  It is necessary in this lab to understand the difference between acids and bases.  One definition is that acids produce H+ in aqueous solution, and bases produce OH–.  Hydrochloric acid and sodium hydroxide combine together in a double displacement reaction to produce a neutral solution.  Hydrochloric acid is actually hydrogen chloride gas in aqueous solution.  We will also use aqueous sodium hydroxide.

Square brackets are used to indicate concentration:  [H+] means “concentration of the hydrogen ion, in moles per liter.”

An indicator is a substance that changes color depending on exactly how acidic a solution is.  You may be familiar with litmus paper, but there are more precise ones available.  In this lab you will use Phenolphthalein ("FEEN-ul-thay-leen") as your indicator.  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.

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. Practice using the valve control on the buret, by allowing some of the solution out a drop at a time into a beaker.  This will also ensure that the buret tip is full of base solution.  Rinse out the beaker when you are done.
  4. Record the volume of base in the buret to the proper number of sig figs.  Record it exactly as you read it; do not subtract from 50.  Also record the exact concentration of the NaOH solution.
  5. From the flask marked “Unknown HCl” in the front of the room, dispense 10.0 mL of hydrochloric acid solution to your beaker.  Use a volumetric pipet to get exactly 10.0mL.  Add two or three drops of indicator to the beaker.
  6. 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.
  7. 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
Vi of HCl __________ __________ __________ mL
Vf of HCl __________ __________ __________ mL
Vi of NaOH __________ __________ __________ mL
Vf of NaOH __________ __________ __________ mL
[NaOH] __________ __________ __________ M
3.
Results Trial #1 Trial #2 Trial #3
V of HCl used __________ __________ __________ mL
V of NaOH used __________ __________ __________ mL
mmol NaOH used __________ __________ __________ mmol
mmol HCl used __________ __________ __________ mmol
[HCl] __________ __________ __________ M
4. Calculate the average concentration of the acid.  Part of your grade will be based on how accurate AND precise your results are.  [Attach all sample calculations for Trial #1!]
5. Why are aqueous solutions used in this lab?
6. What did you see as you added each drop of base?
7. How many moles of sodium hydroxide would be required to neutralize 1.5 mole of hydrochloric acid?
8. How many millimoles of acid are present in 150 mL of 2.0 M HCl solution?
9. What type of reaction is represented by your answer to #1 (S, D, SD, DD, C)?

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

[Simple Titration score sheet][MHS Chem page]