GHS Honors Chem Acid Base Titration Lab

7/27/2019 GHS Honors Chem Acid Base Titration Lab

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GHS Honors ChemistryAcid-Base Titration Lab

Name: ____________________________________ Date: _______ Block: _______

PURPOSE: To determine the concentration of a solution of hydrochloric acid by acid-base titration.

INTRODUCTION:

Titration is a laboratory technique by which we can determine theconcentration of an unknown solution using a standard (known)concentration of another solution that chemically reacts with theunknown. This standard solution is called the titrant. We will alsoneed a way to know that the reaction between the unknown solution,and the standard solution, is complete. In acid/base titrations, anindicator, much like those that you have already seen in the lab, areused. When the indicator changes color (colorless to pink, or viceversa), this indicates that the equivalence point is reached. At the end

point:

Ntitrant x Vtitrant = Nunknown x Vunknown

In an Acid/Base titration, the Normality (N) is simply:

N = M x n

where M = the Molarity of the solution, and n = times the number of or equivalents (the #of H+ or OH- ions that dissociate from the acid or base).

For example:

• HCl contains only one H+, so n = 1, and the Normality (N) = the

Molarity (M)• H2SO4 contains two H+, so n = 2, and the Normality (N) = 2 x M

• NaOH contains only one OH-, so n = 1, and the Normality (N) = 1 x M

• Ca(OH)2 contains two OH-, so n = 2, and the Normality (N) = 2 x M

The calculation for an Acid/Base Titration can be reduced to the following:

nAcid x MAcid x VAcid = nBase x MBase X VBase

In this experiment you will titrate a measured volume of HCl with a solution of NaOH of known concentration. The acid and the base react with one another according to theequation:

HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)

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Name: ____________________________________ Date: _______ Block: _______

The HCl is placed in an Erlenmeyer flask, and phenolphthalein indicator is added. TheNaOH solution is added from a buret into the flask containing the acid. During the first

stages of the titration, the NaOH will be completely neutralized, and an excess of acidwill remain. However, eventually there will be a point, the theoretical endpoint, at whichthe acid and the base have neutralized one another exactly, and no more base shouldbe added to the flask.

The phenolphthalein indicator is used to determine experimentally the point, called theexperimental endpoint, at which the base has neutralized the acid. Phenolphthalein iscolorless in acid solution. It turns pink when the acid is completely neutralized and aslight excess of base is present. In this titration, a successful endpoint is achieved if onedrop of base turns the solution in the flask from colorless to pink.

In this experiment, you can use separate burets for the acid and the base. It is

advantageous to use two burets. If you should overshoot the endpoint by adding toomuch base, you will be able to add an additional measured volume of acid from the acidburet. The additional acid will neutralize the excess base, and you can then add morebase to reach a new endpoint.

In this Acid/Base titration, both NaOH and HCl contain only one OH- and H+respectively, so for each, the number of equivalents (n) = 1. Since you will know or measure:

• MNaOH: the concentration of the NaOH solution in moles/liter

• LNaOH: the volume in mL of NaOH used (which you can convert to L)

• LHCl: the volume in mL of HCl placed in the Erlenmeyer flask

you can calculate the molarity of the HCl solution (MHCl):

MNaOH x VNaOH = MHCl x VHCl

MHCl = ( MNaOH x VNaOH ) / VHCl

For example, if 25.0 mL of 0.200 M NaOH is able to neutralize 30.0 mL HCl, we cancalculate the concentration of the acid solution:

MHCl = (0.200 M NaOH x 0.0250 L NaOH) / 0.0300L HClMHCl = 0.167 M

MATERIALS: o 2 -150 mL beakers

o 2 - 50 mL burets

o double buret clamp

o ring stand

o 250 mL Erlenmeyer flask

o 2 - 250 mL beakers

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Name: ____________________________________ Date: _______ Block: _______

o standardized solution of NaOH

o Phenolphthalein indicator

o Unknown solution of HCl

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Name: ____________________________________ Date: _______ Block: _______

PROCEDURE: A. Record all your experimental data in the table.

DATA TABLE

MEASUREMENTS #1 #2 #3

Concentration of NaOH _______mol/L ________mol/L _______mol/L

Initial reading of base buret ________ mL ________ mL ________ mL

Final reading of base buret ________ mL ________ mL ________ mL

Volume of NaOH used ________ mL ________ mL ________ mL

Initial reading of acid buret ________ mL ________ mL ________ mL

Final reading of acid buret ________ mL ________ mL ________ mL

Volume of HCl used ________ mL ________ mL ________ mL

B. Obtain about 120 mL of HCl solution in a 150 mL beaker. Label the beaker.

C. Rinse one buret with approximately 10 mL of the HCl solution, and let the liquid

drain through the buret tip into an empty 250 mL "waste" beaker. Repeat thisprocedure twice more, using new 10 mL samples of HCl solution each time.

D. Refill the buret so that the meniscus of the solution is above the 0 mL mark.Position the buret in a double buret clamp on a ring stand. Let some of thesolution run rapidly from the buret to expel all air bubbles from the tip and to bringthe level of the solution down to the calibrated region of the buret. If there is adrop of solution hanging on the tip of the buret, remove it by touching the drop tothe inside wall of the 250 mL beaker.

E. Hold a piece of white paper behind the meniscus, and read the initial volume of the solution at the bottom of the meniscus. Your eye must be at the same level

as the meniscus.

F. Obtain approximately 120 mL of NaOH in a second labeled 150 mL beaker.Obtain a second buret and repeat Parts C to E, using the HCl.

G. Let approximately 20 mL of HCl to flow from the acid buret into a clean 250 mLErlenmeyer flask. Add two drops of phenolphthalein.

H. Place the Erlenmeyer flask under the tip of the base buret. A piece of white paper placed under the flask will make it easier to see the color changes. While

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Name: ____________________________________ Date: _______ Block: _______

continuously swirling the flask to ensure thorough mixing, run in the NaOHsolution from the buret. Initially, a pink color will appear at the point where the

NaOH comes in contact with the solution in the flask; however, this color disappears quickly. As the endpoint nears, the color will disappear more slowly.Eventually, the NaOH should be added drop by drop until one drop turns theentire solution in the flask pink. This pink color should remain at least 15 secondswhile the solution is being swirled.

I. If you overshoot the endpoint, add more acid from the acid buret until the solutionbecomes colorless again. Then add the NaOH from the base buret until you havethe faint pink endpoint. When you have reached a satisfactory endpoint, read thefinal volume of each buret.

J. Refill each buret and repeat Parts G to I twice more, using clean Erlenmeyer

flasks.

QUESTIONS:

1. Determine the concentration MHCl obtained from each titration. Then average theresults.

2. If 27.31 mL of 0.2115 M NaOH is able to neutralize 37.45 mL of HCl, what is theconcentration of the acid?

3. What volume of 0.117 M HCl is needed to neutralize 28.67 mL of 0.137 M KOH?4. Why does the pink color, which forms at the point where the NaOH comes into

contact with the solution in the flask, disappear more slowly near the endpoint?5. Why is it a good idea to carry out titrations in triplicate?

6. Why might you be told to use distilled water to wash a drop of solution adheringto one of the buret tips into the Erlenmeyer flask?

7. Would the addition of several milliliters of distilled water to the Erlenmeyer flaskduring the titration affect the results of the titration? Explain your answer.

8. If 35.93 mL of 0.1590 M NaOH neutralizes 27.48 mL of sulfuric acid (H2SO4),what is the M H2SO4?

Modified from:

Whitman, R.L. et al. Chemistry Today 1: Laboratory Manual. 1989. Prentice-Hall CanadaInc.,: Scarborough.

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GHS Honors Chem Acid Base Titration Lab

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