Acid Base Titration

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Quantitative Titration

Introduction

Titration is the name given to the process for determining the volume of a solution needed to react with a given mass or volume of some particular chemical sample. This procedure has many applications, but is especially useful in the reaction between an acid and a base. In this investigation you will use a titration to determine the strength of an unknown base. A prepared solution of 1.0 M

HCl (Hydrochloric acid) will be systematically added to the base solution. As the H

+

ion in the acid reacts with the OH of H

+

ions in the base solution, water forms. The titration is complete when the number ions added is exactly equal to the number of OH

– ions that are present in the base. According to the formula,

M

acid

V

acid

= moles of H

+

= moles of OH

=

M

base

V

base

, the Molarity of the unknown base can be determined. The end point of the titration will be signaled by the color change of an indicator.

The indicator used in this titration is called phenolphthalein. Phenolphthalein exhibits a pink color in the presence of a base, but is colorless in the presence of an acid.

Purpose

To titrate samples of bases to the equivalence point.

To determine the Molarity of an unknown base solution.

Materials

Apparatus

Beakers, flasks graduated cylinder

Buret

Ring stand assembly with buret clamp

Procedure

Part 1 - Setup

Safety goggles, Lab apron phenolphthalein indicator solution

1.0 M HCl Acid

0.5 M, 1.0 M, 2M NaOH Base solutions

Using (

M

acid

V

acid

=

M

base

V

base

), calculate the volume of 1.0 M HCl required to titrate 20.0 ml of

0.5 M, 1.0 M, and 2.0 M NaOH to the equivalence point. Show the calculations and record the HCl volumes in Table 1.

1.

Wear safety goggles and a lab apron at all times in the laboratory.

2.

Note:

Be careful handling the acid and base solutions. Any spills should be cleaned up immediately with a wet paper towel. Any contact with your skin should be rinsed thoroughly with generous amounts of water.

3.

Using proper technique, which involves carefully placing the buret stand on the floor, make sure that the buret contains an adequate amount of 1.0 M HCl before beginning each titration. Proper preparation also includes

“clearing the buret” by allowing a small amount of

HCl to run through the valve.

4.

Once the buret has been prepared with acid solution, record the beginning volume of solution. You need not fill it completely to the zero mark.

Note:

Be sure to read the buret at eye-level and to read the bottom of the meniscus of the liquid.

Part 2 - Titration of the Unknown base with the HCl Solution

1.

Clean and rinse thoroughly with deionized water before using a graduated cylinder to measure

exactly

20.0 ml of 0.5 M NaOH solution and transfer it to a clean beaker.

2.

To the NaOH solution, add 3-4 drops of phenolphthalein indicator. Swirl to mix.

Note:

The solution should be pink at this point.

3.

Place a sheet of white paper under the beaker. Place the beaker and paper under the tip of the buret. The white paper will be helpful in detecting the equivalence point of the neutralization reaction.

4.

Open the valve slightly and allow the HCl to flow slowly into the NaOH solution. As the HCl drips into the NaOH,

continuously

swirl the flask to completely mix the acid and base together. Continue dripping the HCl into the flask until the persistence of the pink

JUST

disappears.

Note:

Use the calculations to estimate how much acid will be required for titration. As the equivalence point approaches, add the acid drop by drop while constantly swirling. When the pink has disappeared and the solution

remains

clear, the equivalence point has been reached. Be careful not to add too much acid so as to go past the equivalence point. In case of doubt, the trial may be repeated with new base solution.

5.

Note and record the final volume of the base in the buret in Table 1.

6.

Repeat steps 1-5 twice more, first using the 1.0 M and then the 2.0 M NaOH solutions. Be sure to place the buret stand on the

floor

to reload it with 1.0 M HCl.

7.

Obtain a sample of unknown base (A, B, or C) as directed by the teacher.

8.

Repeat steps 1-5 for the unknown base sample, twice if time allows.

Cleaning Up

1.

The contents of your titration vessels are safe to be rinsed down the drain with water.

2.

Any unused HCl solution remaining in the buret may be left for the next class. If this is the last class, empty the acid into a beaker, add a few drops of phenolphthalein, and titrate to pink with base before rinsing down the drain. Rinse your buret well with water, and leave it clamped upside down to drain.

3.

Wash your hands before leaving the laboratory.

Data Presentation

TABLE 1. Data Presentation

NaOH Base Molarity

0.5 M

1.0 M

2.0 M

Unknown Trial #1

(letter____)

Unknown Trial #2

(letter____)

Calculated V (ml)

1.0 M HCl for titration

--No Data entry--

Buret start V (ml)

1.0 M HCl

Buret end V (ml)

1.0 M HCl

--No Data entry--

Data Analysis

1.

Transpose the “Calculated V (ml) 1.0 M HCl for titration” column from Table 1 to Table 2.

2.

Using the buret start and end data from Table 1, determine the volume of 1.0 M HCl that was added to each of the 0.5 M, 1.0 M, and 2.0 M NaOH samples to reach the equivalence point and record it in Table 2 in the “Experimental V (ml) 1.0 M HCl for titration” column.

3.

Calculate the percent error for each of the above three trials using the formula below.

Show the calculations and record the results in Table 2.

% error = /Experimental V HCl

– Calculated V HCl/ X 100

Calculated V HCl

TABLE 2. Data analysis for the known titrations.

NaOH Base Molarity

0.5 M

1.0 M

2.0 M

Calculated V (ml)

1.0 M HCl for titration

Experimental V (ml)

1.0 M HCl for titration

% error

4.

Construct a computer generated vertical bar graph depicting the experimental amount of

HCl (y-axis) added to each of the three base concentrations of 0.5 M, 1.0 M, and 2.0 M

NaOH (x-axis) to reach the equivalence point.

Extra credit

if the graph shows a second bar for each trial indicating the calculated amount of HCl for titration next to the experimental bar.

5.

Using the buret start and end data from Table 1, determine the volume of 1.0 M HCl that was added in each of the unknown base trials to reach the equivalence point and record it in

Table 3 in the “Experimental V (ml) 1.0 M HCl for titration” column.

6.

Using (

M

acid

V

acid

=

M

base

V

base

), calculate the Molarity of the unknown base. Show the calculations and record the base Molarity in Table 3.

7.

Bribe the teacher to disclose the actual Molarity of the unknown base is and record in

Table 3. Calculate the percent error for each of the unknown trials using the formula below.

Show the calculations and record the results in Table 3.

% error = /Calculated M Base

– Actual M Base/ X 100

Actual M Base

TABLE 3. Data analysis for unknown titrations.

Unknown

Base

Trial 1

Trial 2

Base

Letter

Experimental V (ml)

1.0 M HCl for titration

Calculated M of unknown Base

Actual M of unknown Base

% Error

Conclusions

1.

List at least 3 factors that either did or could contribute to the percent error?

2.

How could this experiment be repeated to increase the accuracy, or in other words, decrease the percent error?

3.

What effect would adding ten drops of phenolphthalein, rather than 3-4 have on the calculated Molarity of the base solution?

4.

Describe an example of an everyday occurrence that is explained by acid and base behavior.

5.

Show the calculations needed to determine the mass of NaOH needed to make 1.0 liter of each of the following solutions: 0.5 M, 1.0 M, and 2.0 M solutions of NaOH.

6.

Show the calculations needed to determine the volume of 12 M HCl needed to make 4.0 L of 1.0 M HCl.

7.

Describe the procedural process involved in making the solutions in question #5.

8.

Describe the procedural process involved in making the solution in question #6.

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