Purpose:   The purpose of this experiment is to determine the molarity of an unknown  concentration of hydrochloric acid.

Hypothesis: If the volume and molarity of base used in the titration is known and the volume of the acid is measured, then the concentration of the acid can be determined by using the dilution formula.

Variables:

IV:  Moles of acid present

DV:  Moles of base used

Control: Accepted molarity of the base

Materials: 

Safety: Be careful working with hydrochloric acid and sodium hydroxide solutions, they can burn you.

Pre Lab Questions:  Answer the following questions in complete sentences, show work for math questions that you may need to answer.  Include appropriate units and significant figures.

1. A neutralization reaction is a double-displacement reaction between an acid and a base to produce water and a salt.

2. In a titration, a solution of known concentration called the standard solution is used to determine the concentration of an unknown solution.

3. A known quantity of one solution is measured out, and the other solution is added from the buret until the two solutions have the same number of moles as each other.

4.  The point at which two solutions have neutralized each other is called the equivalence point of the titration. The point at which the color changes to signal the end of the titration is called the endpoint of the titration. 

5. An indicator is used to mark the endpoint of the titration. Phenolphthalein is an indicator often used in acid base titrations. It is colorless in acid solution and pink in base solution.

6. Label the following diagram: (base of known concentration, buret, acid of unknown concentration, stopcock)

Background:  Titration involves the addition of a solution whose concentration is known to a solution whose concentration is unknown.  The volume of the known solution required to react completely with a known volume of the solution whose concentration is being determined is measured.  An indicator is added to the solution to mark the point at which the two quantities reach equivalence.

In this experiment sodium hydroxide of known concentration will be added to a known volume of hydrochloric acid to determine the molarity of the sodium hydroxide.  Phenolphthalein will be used as the indicator.  Phenolphthalein is pink in a basic solution and clear in an acidic solution.

Objective:  To determine the molarity of an unknown concentration of hydrochloric acid.

Safety:  Wear goggles and aprons – both acids and bases are corrosive and can cause eye and/or skin damage. Do not pour the NaOH above eye level.  Lower the buret below eye level before filling it. Water soluble solutions can be poured down the sink.

Materials:

Ring stand    1 Erlenmeyer flask   

phenolphthalein       2 beakers 

Buret DI water

0.5 M NaOH Funnel          

Clamp                  10 mL syringe        

 unknown acid 100 mL graduated cylinder

Procedure:

Set up the ring stand, buret, and buret clamp.  Make sure that the buret remains vertical at all times. See image to the right. Add some DI water to the buret to check for leaks.  If there are no leaks, drain the DI water completely from the buret. Carefully add the NaOH to the buret.  You will need to drain some of the NaOH from the buret to fill the tip.  Fill the buret so that the meniscus of the NaOH is sitting on the 0.00 mL mark with the tip filled.  Using the graduated cylinder add about 60 mL of DI water to a 250 mL Erlenmeyer flask (or 20 mL to a 125 mL Erlenmeyer flask). Using the syringe add exactly 10 mL of the acid unknown to the water in the Erlenmeyer flask.  Make sure that you write down the letter of your unknown. Add 2 – 3 drops of phenolphthalein to the acid solution in the Erlenmeyer flask. Place the Erlenmeyer under the buret.  Place a sheet of white paper under the Erlenmeyer.

You are now ready to begin titrating.  Carefully add about 1 mL of the NaOH from the buret to the Erlenmeyer flask and swirl to mix. Continue adding 1 mL increments until you begin to see a pink color.  The pink color should appear briefly and then quickly disappear. Once you have seen the pink color, begin adding the NaOH slowly and in smaller amounts.  Remember to swirl the flask after each addition of  NaOH.  As you near the endpoint, the pink color should stay longer, but will still disappear when the flask is swirled.  The longer the pink color stays, the smaller your next increment of NaOH should be.  You want to eventually be adding the NaOH one drop at a time until the palest pink color appears and stays.  

When the pale pink color appears and stays you have reached the endpoint.  Record the buret reading to the nearest 0.01 mL. Clean out the Erlenmeyer flask and repeat steps 4 – 10 twice more  (the Erlenmeyer does not need to be dry, only clean).  The solution in the Erlenmeyer can be poured down the drain.  You will not need to add more NaOH to the buret for the second trial.  Before you start the third trial, make sure that you have enough NaOH in the buret to complete the titration (use your values from the first and second trials to estimate the amount needed for the third trial). Clean out the Erlenmeyer and dispose of any remaining acid or base according to your teachers instructions.