1180-Exp-13-Titration.pdf

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Health Science Chemistry I

CHEM-1180

Laboratory Experiment No. 13

Acid-Base Titration(Revised 01/20/2014)

Titration is defined as the process of determining the concentration of a solution by comparing it to theknown concentration of another solution. In an acid-base titration, the point at which moles of acid are

equivalent to moles of base is the equivalence point. This is expressed algebraically as:

na(mol) = nb(mol) (Equation 1)

In the titration of acetic acid, the active ingredient in vinegar, all reactants and products are colorlessand the equivalence point is invisible. A small amount of added acid-base indicator allows

visualization of the equivalence point. Phenolphthalein, the indicator used in this experiment, is

colorless in an acidic solution and turns pink when the acid is neutralized and the solution turns

slightly basic. (When a weak acid is titrated with a strong base such as NaOH, the solution at the

equivalence point is slightly basic due to hydrolysis of acetate ion.) The point at which the indicatorturns color is the endpoint. The visual endpoint may not exactly correspond to the equivalence point,

but selecting the correct acid-base indicator gives the closest match between the endpoint and the

invisible equivalence point.

The concentration of a base titrating solution can be determined in one of two ways:

1. By using the base solution to titrate a standard acid solution of precisely known concentration, such

as commercially available 0.100 M HCl.

2. By using the base solution to titrate a known amount of a primary standard, a solid acid that can

be weighed easily and that reacts with base in a known stoichiometric manner.

We will do the second procedure and use a solid acid primary standard to standardize the NaOH

solution. Requirements for a primary standard acid are:

It must exist in very pure form, preferably as a crystalline solid.

It must be chemically stable, must not react with atmospheric oxygen or absorb water from the air.

It must have a high formula mass to minimize the effect of weighing errors.

It must react in a known manner with the solution to be standardized.

A commonly used primary standard for standardizing base solutions is the monoprotic weak acid

potassium hydrogen phthalate, HKC8H4O4, also called acid potassium phthalate. Its name may be

abbreviated as KHP where P stands for phthalate, not phosphorus. The formula mass of KHP is204.2234 g/mol. Verify this for yourself as an exercise. KHP reacts with sodium hydroxide in a one

to one molar ratio according to the equations:

KHC8H4O4(aq) + NaOH(aq) NaKC8H4O4(aq) + H2O(l) (Equation 2)or

KHP(aq) + NaOH(aq) NaKP(aq) + H2O(l) (Equation 3)


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Equation 1 is the starting point of this exercise since moles of acid equal moles of base at the

equivalence point. In this case, moles of the solid acid, symbolized by na, are calculated from the mass

of KHP weighed out (between 0.6 and 0.7 g) and the KHP formula mass, 204.223 g/mol as shown inEquation 4.

a

mass acid(g)n (mol)

formula mass(g / mol) (Equation 4)

At the endpoint, moles of acid equal moles of base, but moles of base solution, nb(mol), also equal the

base concentration times the volume of solution used (Equation 5).

na(mol) = nb(mol) = Mb(mol/L) x Vb(L) (Equation 5)

You will make up an approximately 0.1 M NaOH solution from stock 6 M NaOH. You will then use

this base solution to titrate a known mass of the standard acid and calculate the actual molarity of the

base, Mb, using Equation 6. Then you will use the base solution to titrate an unknown acid solution

(vinegar) and determine its molarity, Ma, using Equation 7. Base volumes will be measured with aburet marked in 0.1 mL intervals and read to 0.01 mL. The unknown acid will be dispensed with a 5

mL pipette. For keeping track of significant figures, assume that the delivered pipette volume is

precise to 0.01 mL, that is, that the pipette delivers 5.00 mL of solution.

The concentration of the base titrating solution is calculated by combining Equations 4 and 5 into

Equation 6 and solving for the base concentration, Mb. Take into account the fact that the fact that

molar concentration is expressed as mol/L while the buret measures base solution volume in milliliters.

mass acid(g)

formula mass(g / mol) = Mb(mol/L) x Vb(L) (Equation 6)

After you determine the exact NaOH solution molarity by titration with KHP, you will use thatconcentration to determine the molarity of acetic acid in an unknown vinegar sample by solving for M a

in Equation 7, derived by substituting n(mole) = M(mol/L) x V(L) into each side of Equation 1.

MaVa = MbVb (Equation 7)

Note: The standardization titrations do not necessarily have to be performed before the vinegar

titrations. Either set of titrations may be done first, whichever is more convenient, depending on the

availability of balances and KHP supply bottles in the laboratory.

Experimental

Preparation of NaOH Titrating Solution: Be sure your safety goggles are properly fitted to yourface. Make an approximately 0.1 M NaOH solution by dilution from 6 M NaOH stock solution as

follows: Dispense about 11 mL of 6 M NaOH into a 500 mL Florence flask. Fill the flask halfway

with distilled water and swirl the flask for 15 s to mix the contents thoroughly. When the solutionappears completely homogeneous, fill the flask to the bottom of the neck with distilled water for a total

volume of about 500 mL. Place a rubber stopper securelyin the neck, support it with the palm of your

hand, invert the flask and swirl it for 5 s. Right the flask and repeat this step two more times to ensure


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the solution is uniform. If you see swirls of inhomogeneity, the solution is not yet well mixed. Keep

the ~0.1 M NaOH sealed with the rubber stopper. Remember, the concentration of base in this

solution is notwell known. Its actual concentration will be determined by titration. You will use thissolution for all subsequent titrations. Do not discard it until the end of the experiment.

NaOH is never used as a primary titration standard because the compound cannot be weighedaccurately. NaOH pellets exposed to the air immediately absorb water and carbon dioxide so their

purity is always in doubt. Moreover, NaOH solutions undergo slow conversion to sodium carbonateunless they are carefully protected from atmospheric CO2.

Filling Buret with Base Solution: Obtain a 50 mL buret. If the buret is clean and dry, there is no

need to wash the buret or rinse it with tap water. Insert a stopcock and tip into the buret bottom. If

they do not fit tightly, match a different stopcock with your buret. Rinse the buret twice with 5 mLportions of your 0.1 M NaOH solution, drain the solution through the tip and discard it. Support the

buret vertically with a buret clamp over a sink so the top of the buret is at or below your eye level. Fill

the buret with NaOH solution nearly to the top. Open the stopcock and run some base solution out

rapidly while tapping the stopcock to dislodge any bubbles hidden inside. Adjust the volume of baseso that it is at, or slightly below, the zero mark. It is not necessary to get the initial volume to exactly

zero. Record the initial volume of the NaOH solution to the nearest 0.01 mL. (Note: If the 50-mLburet is filled to the very top graduation, the initial volume should be read as 0.00 mL, not50.00 mL.)

NaOH Solution Standardization: Using small beakers or plastic weigh boats, weigh four 0.6 to 0.7 g

samples of KHP on an electronic balance to 0.001 g. Take advantage of the convenient electronicbalance tare function. Place a weighing container on the balance pan and push the Tare or T button.

The balance display will read 0.000 g, showing the balance considers the container to have no mass.

The mass of any material you put in the container will be displayed directly, eliminating the need for

two separate weighings and a subtraction step to get the mass of the chemical. (If the balance displayreads to only two decimal places, inform the instructor.)

Quantitatively transfer each weighed KHP sample to a labeled, clean, but not necessarily dry 125 or250 mL Erlenmeyer flask. Pour the solid through a wide-stem funnel into the Erlenmeyer flask and

wash all traces of the chemical out of the weighing container and funnel with distilled water from your

wash bottle. Add about 50 mL of distilled water and two drops of phenolphthalein indicator to each

Erlenmeyer flask and swirl briefly. (The KHP will not dissolve completely until you add NaOHsolution.) Titrate each KHP solution with NaOH solution as follows:

For easy visualization of the endpoint, place a sheet of white paper on the ring stand base under thetitration flask. Record the initial buret reading. Lower the buret so the tip is insidethe Erlenmeyer

flask and run base solution rapidly into the flask. If you are right handed, grip the neck of the

Erlenmeyer flask with the fingers of your right hand and swirl the solution rapidly to mix the acid and

base. Meanwhile operate the stopcock with your left hand. As the endpoint approaches, all the solidKHP will dissolve. The color change from clear to pink will persist longer where the base enters the

solution. The final addition of base should be slow, one drop at a time to avoid overtitration. Rinse

the upper flask walls several times during the titration with a few milliliters of distilled water fromyour wash bottle to ensure all the acid and base mix in the flask. Distilled water contains equal (and

almost negligible) amounts of hydrogen and hydroxide ion and will not affect the titration results. You

may also rinse the buret tip to remove hanging drops of base solution. Do not rinse excessivelybecause that will remove sodium hydroxide solution from inside the tip.


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The titration endpoint occurs when the solution changes from colorless to a persistent pale pink color.

Sometimes less than half a drop of base brings this about. When you believe you are near the

endpoint, transfer less than one drop of base from the buret tip to the wall of the flask, rinse it downwith distilled water and swirl. Repeat this step until you reach the visible endpoint. Do several more

titrations until you have three that agree to within a few percent. Since your KHP masses are likely to

be different, check for consistency of your titrations by dividing the volume of base solution dispensedby the KHP mass (no units are necessary). Write the calculated ratios with the correct number of

significant figures in the blank space to the right of the titration data. This volume/mass ratio shouldbe nearly constant for all titrations.

If you add base solution beyond what is needed to get the first pale pink color, you have overtitrated.

A pale pink endpoint is best; medium pink is acceptable. Deep pink means you have added too much

base. Do not use overtitrated results in your calculations. If you use more than 24 mL of base solutionfor a titration, fill the buret before the next titration so you will not run out of base partway through the

next procedure.

Titration of Unknown Acid: Obtain an unknown vinegar solution and record its number on your datasheet. Rinse a 5 mL pipette two times with a small amount of your unknown acid and discard the

rinsate down the drain. Pipette 5 mL aliquots of your unknown acid into four separate clean (but notnecessarily dry) Erlenmeyer flasks as follows: Fill the rinsed pipette by suction using a rubber bulbuntil the acid solution level is about 2 cm above the upper stem fill line. Never pipette by mouth. Let

in small amounts of air with the bulb or your finger tip until the bottom of the meniscus just touches

the fill line. Move the pipette to an Erlenmeyer flask and drain the pipette into the flask with thepipette tip held against the inside of the neck. Wait 2 s after the flow stops, touch the pipette tip to the

surface of the liquid and remove the pipette from the flask. Do not blow out the liquid remaining in

the tip. Volumetric pipets are calibrated to deliver, not to contain, the specified amount. Add two

drops of phenolphthalein indicator to each flask and titrate each solution with your standard NaOHsolution to a persistent pale pink endpoint. Repeat the titrations until you have three reliable results

(pale pink endpoints with base volumes that agree closely) from which to calculate the average

molarity of your acetic acid sample. (Since the volume of unknown acid solution you pipette is thesame each time, the NaOH solution volumes should be the same for each titration. There is no need to

calculate a base volume to acid ratio as before.)

Cleanup: When you finish titrating, remove the stopcock and rinse the buret, stopcock and tipthoroughly with tap water. Sodium hydroxide solution corrodes glass and will etch the buret if

allowed to remain inside. After you rinse the buret, your instructor may tell you to place the buret in a

special buret cleaner. Return your pipette and unknown bottle to the stock room. Wipe up any spilledKHP from the balance and surrounding area. Wipe your benchtop down with a wet sponge and wash

your hands before you leave the room.

Safety: The stock 6M NaOH solution is extremely corrosive and dangerous to the eyes. Chemicalsplash goggles and a waterproof apron must be worn at all times during this and all chemistry

experiments, from the very beginning to the very end of the time you spend in the laboratory.

Waste Disposal: All spent titration solutions are neutralized and may be safely rinsed down the drain

with water. Your instructor will tell you how to dispose of unused 0.1 M NaOH titrating solution.


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CHEM-1180


Acid-Base Titration

Data Page for Sodium Hydroxide Standardization Titrations

Standardization Titration # 1Mass of KHP

Final NaOH Volume

Initial NaOH Volume

NaOH Titrating Volume

Endpoint Color

Standardization Titration # 2

Mass of KHP

Final NaOH Volume

Initial NaOH Volume


Endpoint Color


Mass of KHP

Final NaOH Volume

Initial NaOH Volume


Endpoint Color


Mass of KHP

Final NaOH VolumeInitial NaOH Volume


Endpoint Color


Mass of KHP

Final NaOH Volume

Initial NaOH Volume


Endpoint Color

Standardization Titration # 6Mass of KHP

Final NaOH Volume

Initial NaOH Volume


Endpoint Color


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CHEM-1180


Acid-Base Titration

Data Page for Unknown Vinegar Solution Titrations

Unknown Vinegar Solution Number:________________

Vinegar Titration # 1

Volume Acid Pipetted

Final NaOH Volume

Initial NaOH Volume


Endpoint Color



Final NaOH Volume

Initial NaOH Volume


Endpoint Color



Final NaOH Volume

Initial NaOH Volume


Endpoint Color



Final NaOH Volume

Initial NaOH Volume


Endpoint Color



Final NaOH Volume

Initial NaOH VolumeNaOH Titrating Volume

Endpoint Color


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CHEM-1180


Acid-Base Titration

Report

Clearly show the equations, setups and steps for each calculation.1. For each of your three best standardization titrations, use Equation 4 to calculate the moles of

KHP weighed out. (3 points)

2. For each of your three best standardization titrations, solve Equation 6 to calculate the molarity

of your NaOH titrating solution. (6 points)

3. Calculate the average molarity of the NaOH titrating solution from your three best trials.

(2 points)


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4. Calculate the average volume of NaOH solution used in your three most consistent titrations of

the unknown acetic acid (vinegar) solution. (1 point)

5. Write your Unknown Vinegar Solution Number here:__________

Use Equation 7 to calculate the molarity of your unknown acetic acid solution. (4 points)

6. Calculate the mass of sodium hydroxide in one liter of your NaOH titrating solution. (2 points)

7. Calculate the percent (w/v) concentration of your unknown acetic acid solution, that is, the

mass of acetic acid in 100 mL of solution. (2 points)


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CHEM-1180


Acid-Base Titration

Prestudy

1. A 0.7483 gram KHP sample requires 14.98 mL of a NaOH solution to neutralize it using

phenolphthalein as an indicator. What is the molarity of the NaOH solution? (4 points)

2. A 10.00 mL sample of acetic acid solution is titrated with 35.87 mL of 0.217 M NaOH solution.

Calculate the molarity of the acetic acid solution. (4 points)

3. Why is phenolphthalein the indicator of choice for the titrations in this experiment? (2 points)

1180-Exp-13-Titration.pdf

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