Experiment No. 8 Acid-base Titration

Experiment No. 8 Acid-Base Titration

De Jesus, Nicolle Dela Cruz, Maria Margarita Dela Cruz, Paula Mae Dongon, Marinelle

1NUR-3 Group 4

Introduction The reaction between a base and an acid involves neutralization. It can be written as: Acid + Base

Salt + H 2O

This reaction is used in titration, a quantitative analytical method. The objective is to determine the concentration of an acid or base. In acid-base titration, the analyte is made to react with the standard solution. The analyte is a measured volume of unknown solution. On the other hand, the standard solution is a solution whose exact concentration is known. The analyte is placed in an Erlenmeyer flask and the standard solution is gradually added from a burette until complete neutralization has been reached. The equivalence point is the point at which all the acid (or base) in the sample has completely reacted with the standard solution of base (or acid). The equivalence point is also the endpoint of titration. The endpoint is indicated by the change in colour of an appropriate indicator. The appropriate indicator for titration depends on the strength of the acid and the base reacting. For titrations of strong acids and bases, the pH at the endpoint is 7.0. The indicator should change its hue at about pH 7.0. For weak acids with strong bases, the indicator should change colour at a pH above 7.0. On the other hand, strong acids with weak bases need an indicator that changes colour below 7.0. Upon reaching the endpoint, the number of equivalents of the acid is equal to the number of equivalents of the base. Equivalents are the quantities of substances that have the same combining capacity in chemical reactions. In a neutralization reaction, it

is the mass of acid that yields 1 mol of H + or the mass of base that reacts with 1 mol of  H+. At the endpoint of titration, VaNa = VbNb Where: Va = Volume of an acid

V b = Volume of a base

Na = Normality of an acid

N b = Normality of a base

Normality is the number of equivalents of a solute per liter of solution. N = no. Of equivalents of Solute Volume of Solution in Liters

Since the no. Of equivalents is equal to the grams of solute/gram-equivalent weight (GEW), N = g of solute GEW (Lsolution )

GEWacid = molar mass of the acid / no. Of replaceable H + GEWbase = molar mass of the base / no. Of OH - ions If the given concentration of the standard solution is normality, the concentration of the unknown solution can be solved for either in one of the following: Normality, Molarity, and Percentage. For this experiment, the percent concentration of the unknown solution will be determined.

Objectives •

Students should be able to perform an acid-base titration.

•

They should be able to use the gathered data to solve for the concentration of an unknown solution.

•

The students should be able to work efficiently and use critical thinking.

Observations/Results A.

Analysis of commercial Vinegar 1 (Datu Puti) – Group 5 Specific Objective: To determine the concentration of vinegar by performing an acid-base titration.

NaOH: 0.1229 N

Final Reading Initial Reading Volume NaOH used:

Trial 1 47.2 mL 0 47.2 mL

Trial 2 46.6 mL 33.4 mL 13.2 mL

Trial 1 8.5 mL 0 mL 8.5 mL

Trial 2 14.5 mL 8.5 mL 6 mL

.35 4.12

.34 5.67

Sample Used: Vinegar 

Final Reading Initial Reading Volume Sample used:

Weight of Acid Sample %w/v of Acid in Sample Mean % w/w: 4.9% B.

Analysis of commercial Vinegar 2 (Silver Swan) – Group 1 Specific Objective: To determine the concentration of vinegar through acid-base titration.

NaOH: 0.1229 N

Final Reading Initial Reading Volume NaOH used: mL Sample Used: Silver Swan Vinegar 

Trial 1 29.2 mL 0 mL 29.2 mL

Trial 2 45.6 mL 14.9 mL 30.7

Final Reading Initial Reading Volume Sample used:

Weight of Acid Sample %w/v of Acid in Sample Mean % w/v: 4.5% C.

Trial 1 45mL 50mL 5mL

Trial 2 40mL 45mL 5mL

0.22 g 4.4 %

0.23 g 4.6%

Analysis of commercial Fruit juice – Group 2 Specific Objective: To determine the concentration of fruit juice by performing an acid-base titration.

NaOH: 0.1229 N

Final Reading Initial Reading Volume NaOH used:

Trial 1 3.5 mL 0 mL 3.5 mL

Trial 2 32.8 mL 29.9 mL 2.9 mL

Trial 1 35 mL 25 mL 10 mL

Trial 2 45 mL 35 mL 10 mL

0.028 g 0.28%

0.023 g 0.23%

Sample Used: Fruit juice

Final Reading Initial Reading Volume Sample used:

Weight of Acid Sample %w/v of Acid in Sample

Mean %w/v: 0.26%

D.

Determining the Strength of Antacid

Specific Objective: To be able to observe changes in acid-base titration and gather data to determine the concentration of an Antacid Sample Used: NaHCO 3 No. Of mg: 300 mg HCl: 0.1141 N (For Mefenamic Acid: NaOH: _____ N)

Final Reading Initial Reading Volume used:

Trial 1 30.9 mL 4 mL 0.0269 L

Trial 2 25 mL 0 mL 0.025 mL

NaOH: 0.1229 N (For Mefenamic Acid: _____ N)

Final Reading Initial Reading Volume used:

Trial 1 31.2 mL 28 mL 3.2 x 10 -3 L

Trial 2 7 mL 3.5 mL 3.5 x 10 -3 L

Equiv. Of Base/Acid (step

3.07 x 10 -3

2.85 x 10 -3

2) Equiv. Of Base/Acid Not in

3.9 x 10-4

4.3 x 10-4

tablet (step 3) Equiv. Of Base/Acid in

2.68 x 10 -3

2.42 x 10 -3

0.3g

0.3 g

75.04%

67.76%

tablet (step 2 – step 3)

Weight of Base/Acid Sample %w/w of Base/Acid in Sample Mean % w/w: 71.4%

E. Determining the Strength of Mefenamic Tablets:

Specific Objective: To be able to observe changes in acid-base titration and gather data to determine the concentration of a Mefenamic tablet. Sample Used: Generic Mefenamic acid tablet No. Of mg: 500 mg

HCl: 0.1141 N (For Mefenamic Acid: NaOH: _____ N)

Final Reading Initial Reading Volume used:

Trial 1 40.0 mL 24.8 mL 15.2 mL

Trial 2 19.4 mL 2.5 mL 16.9 mL

NaOH: 0.1229 N (For Mefenamic Acid: _____N) Trial 1 28.8 mL 3.7 mL 25.1 mL

Trial 2 24.8 mL 0.0 mL 24.8 mL

Equiv. of Base(step 2) Equiv. of Acid Not in tablet

3.08 x 10 -3 1.73 x 10 -3

3.05 x 10 -3 1.93 x 10 -3

(step 3) Equiv. of Acid in tablet (step

1.35 x 10 -3

1.12 x 10 -3

0.5 g 65.07 % Mean % w/w: 59.53%

0.5 g 53.98 %

Final Reading Initial Reading Volume used:

2 – step 3)

Weight of Acid Sample %w/w of Acid in Sample

Discussion

Vinegar (CH 3COOH) The volume of the NaOH and vinegar used was obtained by subtracting the initial reading in the burette from the final reading. The weight of the acid sample is calculated by multiplying the Normality of the base to the Volume of  Base in Liters and to the GEW (gram-equivalent weight). The % m/v of acid in sample was obtained by dividing the mass of the sample to the volume sample in millilitres and multiplying it to 100. In getting the mean % w/v, we added the two percents and divided them to two. Antacid (NaHCO 3) An antacid tablet was weighed and recorded as 300 mg. It was placed in an Erlenmeyer flask. HCl, the titrant, was added. The amount was recorded as 0.0269 L. The antacid was made to dissolve with the titrant using a stirring rod. The stirring rod was rinsed within the flask. Two drops of the indicator  was mixed in. The solution should be permanently colourless by now. If a pink color developed, more HCl should be added. The added HCl should be recorded. The number of equivalents of excess titrant was then calculated to be 3.07x10-3 by multiplying the volume in liters by the normality of HCl which is 0.1141 N. The excess titrant or acid was then neutralized using a backtitrant NaOH and the recorded amount was 3.2x10 -3. It was added until a permanent faint pink color was evident. The number of equivalents of the backtitrant was calculated as 3.9x10 -4 by multiplying the amount of backtitrant with the normality of NaOH which is 0.1229 N. % mass was then calculated by subtracting Eq of titrant minus Eq of backtitrant. This was then multiplied

by GEW of NaHCO 3 which is 84. Dividing it by the mass of the tablet in g which is 0.3 g and multiplying it by 100, the answer is 75.04% Another trial was then made and the calculated %mass was 67.76%. The mean of these is 71.4% Mefenamic Acid A generic mefenamic acid tablet was weighed and recorded as 500 mg. It was placed in an Erlenmeyer flask. Sodium Hydroxide (NaOH), the titrant, was added. The amount was recorded as 0.0251 L. It was made to dissolve with the titrant using a stirring rod. The stirring rod was rinsed within the flask. Two drops of the phenolphthalein indicator was mixed in. The solution should be permanently pink by now. If a pink color developed, more NaOH should be added. The added NaOH should be recorded. The number of equivalents of  excess titrant was then calculated to be 3.08x10 -3 by multiplying the volume in liters by the normality of NaOH which is 0.1229 N. The excess titrant or base was then neutralized using a backtitrant HCl and the recorded amount used was .0152L . It was added until a permanent light pink color was evident. The number of equivalents of the backtitrant was calculated as 1.73x10 -3 by multiplying the amount of backtitrant with the normality of HCl which is 0.1141 N. % mass was then calculated by subtracting Eq of titrant minus Eq of  backtitrant. This was then multiplied by GEW of mefenamic acid which is 241. Dividing it by the mass of the tablet in g which is 0.5 g and multiplying it by 100, the answer is 65.07% Another trial was then made and the calculated %mass was 53.98%. The mean of these is 59.53%

Study Guide Questions and Answers 1. Discuss briefly why the following procedure must be observed during titration: a. Acids are placed in burettes with glass stopcocks while bases are placed in rubber-tipped burettes. Acids are placed in burettes with glass stopcocks to prevent corrosion. Acids corrode rubber. On the other hand, bases are placed in rubbertipped burettes to prevent formation of sticky substance due to reactions of glass and a base. b. The burette is rinsed with the solution to be placed in it after rinsing with distilled water. This is done to prevent the solution from getting stuck to the sides of the burette. c. Before the initial reading is taken, the glass tips of the burette should be completely filled with the solution. The glass tips should be completely filled because the liquid that it will hold is also included in the measuring the acid and taking its initial reading. d.

Care should be taken not to spill any liquid from the flasks or burettes before the end point is reached. Care should be taken to prevent spilling of any liquid because it will have a greatly affect the weight and concentration of the liquids present. The experiment, therefore, cannot be considered accurate.

e. Rinsing the sides of the Erlenmeyer flask with the distilled water before the end point is reached. We rinse the sides of the Erlenmeyer flask so all the analyte will react with the standard solution. 2. What is the effect of the following condition on the calculated percentage of the acid sample? (Increased, decreased or No effect at all). a. The glass tips of the burettes were not filled with the sample solutions when the titration was started. Decreased. It’s because when the NaOH decreases, the percentage of  acid will decrease. b. More distilled water was added to the flask than what was required in the procedure. No effect because distilled water is neutral that’s why it won’t affect the acidity nor the basicity of the sample solution. c. The base burette was not rinsed with the standard base before filling it with the standard solution. No effect because the basic solution and the acidic solution will not mix. d. The standard base was allowed to flow down the sides of the flask leaving some drops unreacted until the endpoint is reached. Increase because an increase of NaOH will increase the percentage of  acid sample.

e. Just before the end point is reached, the sides of the flask were rinsed with is much distilled water that droplets of the standard solution adhering it the side combined with the solution being titrated. Increased. The droplets of NaOH mixed with the acid sample making it more basic. 3. Write the general equation for an acid-base titration. Acid + Base 4.



Salt + H2O

Describe the action of mefenamic acid and NaHCO 3 in our body. Mefenamic acid works by blocking the action of a substance in the body called cyclo-oxygenase (COX). Cyclo-oxygenase is involved in the production of  various chemicals in the body, some of which are known as prostaglandins. Prostaglandins are hormones produced by the body in response to injury and certain diseases and conditions, and cause pain, swelling and inflammation. Mefenamic acid blocks the production of these prostaglandins and is therefore effective at reducing inflammation and pain. Mefenamic acid may also work by preventing the action of prostaglandins after they have already been formed.

Sodium Bicarbonate (NaHCO 3) is used as an antacid to treat acid indigestion and heartburn. An aqueous solution can be administered intravenously for cases of acidosis (low pH), or when there are insufficient sodium or bicarbonate ions in the blood. Here’s an example of its chemical reaction: HCl+NaHCO 3 NaCl+CO2+H2O

5. The electrolyte in body fluids is measured in terms of an equivalent at milliequivalent. Define an equivalent of an ion. Give the formula needed to calculate the equivalent weight of an ion.

An equivalent (Eq) of an ion is the molar amount of that ion needed to supply one mole of positive or negative charge. GEWacid = molar mass of the acid / no. Of replaceable H + GEWbase = molar mass of the base / no. Of OH - ions Eq weight= molar mass/ionic valence 6.

What is the relationship between an equivalent and a milliequivalent. How is a concentration of milliequivalents per liter converted to millimoles per deciliter? Equivalent (Eq) of an ion is the molar concentration of that ion needed to supply one mole of (+) or (-) integer. A small unit is used in specifying electrolyte concentrations, which is the milliequivalent unit. The reason why is because of  the relatively low concentrations of ions present in body fluids. 1 milliequivalent = 10 -3 equivalent

7.

One of the cations present in the body is Ca 2+. How many milliequivalents of Ca 2+ are present in 100 ml of a 0.1% (w/v) Ca 2+ solution?

8.

In body fluids, concentrations of electrolytes are expressed as mEq.L. A patient’s chart shows that his serum chloride ion concentration is 94 mEq.L. What is this concentration on millimoles/liter. 94 mEq/L x 1Eq/1000mEq x 1 mole/ 1 Eq x 1000 mmol/ 1 mole =

9.

94 mmol/L

Solve the following problem: Aspirin contains a monoprotic called acetylsalicylic acod. Its molecular formula is C 9H8O4. A certain pain reliever was analyzed by dissolving 0.250 g of the sample in 20.0 mL of 0.10 N NaOH and bactitrating the excess base using 0.120 N HCl solution. The backtitration process required 805 mL of HCl. What is the 5 by mass of acetylsalicylic acid in the pain reliever?

References 1. Chemical Technician’s Ready Reference Handbook. Third Edition. (Shugar & Ballinger) – pp. 605-610, 620-627 2. A laboratory for General, Organice, and Biochemistry. Fourth Edition. (Henrickson, Byrd, & Hunter) – pp. 172-181 3. Chemistry: The Molecular Nature of Matter and Change. Fourth Edition. (Silberberg) – pp. 144-149 4. Exploring General, Organic, and Biological Chemistry. Philippine Edition. (Stoker)  – pp. 376-379