TAB2, Group 7, Mr. Julius Andrew P. Nuñez March 1, 2011
Experiment 7: Acid, Bases and Salts Espiritu, Adelaine Joy F. Galvan, Ma. Danica Katrina P. I. Abstract
Experiment 7 entitled Acid, Bases and Salts, is divided into three main parts. The first part involves classifying a solution whether it is an acid, base or salt. This includes identifying the pH level of the given solutions. The second part determines the conductivity of the solution and tells whether it is an electrolyte or not. The last part includes the calculation of the molarity of an unknown acid using the computed values. The experiment is mainly done by testing the solutions using the given indicators. Correct computations must be applied in order to get accurate results in the calculation of molarity. The experiment requires the ability of the students to note the observations regarding the effects of adding the indicators in the solutions. It also requires the skill of proper titration in order to get the perfect results. As we encounter different solutions in our everyday lives, it is important to know its classification and concentration. This experiment will show the differences of an electrolyte from a non-electrolyte as well as the distinction of a solution as to whether it is an acid, base or salt. Experiment on acids, bases and salts will also explain how to get the weight, volume and concentration of particular solutions. II. Keywords: electrolytes, acids, bases, salts, pH, titration III. Introduction Solutions are homogenous mixtures of two or more substances, which may be solids, liquids, or gases. It may be classified as an electrolyte, in which the interaction between ions leads to the formation of ion pairs, or as a non-electrolyte which does not contain ions. Electrolyte solutions have the ability to conduct electricity while non-electrolyte solutions cannot conduct electricity. Electrolyte solutions may be further classified into acids, bases, or salts. An acid is a substance that yields hydrogen ions when dissolved in water. Acids have a sour taste. It changes the color of litmus from blue to red. Aqueous acid solutions conduct electricity. A base is a substance that yields hydroxide ions when dissolved in water. Bases have a bitter taste and feel slippery. It changes the color of litmus from red to blue. Aqueous base solutions conduct electricity. A salt is an ionic compound made up of a cation other than H+ and an anion other than OH- or O2-. It is generally produced together with water during a neutralization reaction, a reaction between an acid and a base. The acidity of an aqueous solution is expressed as its pH which is defined as the negative logarithm of the hydrogen ion concentration. In determining the concentration of a solution, a process called titration is done. Titration involves a solution of accurately known concentration called a standard solution that is added gradually to another solution of unknown
Chem 14.1, Acids, Bases and Salts
concentration until the chemical reaction between the two solutions is complete. IV. Experimental A. Electrolytes Five drops of 0.1 M solutions of NaOH, NH4Cl, HC2H3O2, NaCl, C2H5OH, C12H12O11, HCl, NH4OH, HOAc + NaOH, HCl + NaOH, NH4OH + HCl and distilled water was placed in separate test tubes and was tested using the following indicators: litmus paper, phenolphthalein and congo red. Color was taken note of and the substances were classified into acids, bases and salts. The pH oh each substance was obtained using a pH paper. Equal volumes, specifically 1 ml of 1 M HCl and 1 M NaOH was mixed in a test tube. Indicators were used to test the mixture. This procedure was repeated by using 1 M acetic acid in place of HCl. B. Conductivity Test The conductivity of the above solution was tested using the conductivity apparatus. C. Preparation of 1 M NaOH using NaOH pellets The weight of NaOH needed to prepare 100.00 mL of 1 M NaOH was calculated. The calculated amount of pellets was weighed in a watch glass and was dissolved in 50 mL water.
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D. Preparation of 0.1 M NaOH from available concentration of NaOH The volume of NaOH from available concentration needed to prepare 100 mL of 0.1 M NaOH was determined. The measured volume to from procedure A was dilute to 100.00 mL distilled water in a volumetric flask.
Upon the addition of phenolphthalein, bases turned into pink while colorless for the acids and neutral solutions. For congo red, bases and neutral solutions turned into red while acids turned into violet. To know the strength of the acidty/basicty of the solution, the strength of conductivity must be observed. The following were the obtained results from the conductivity test:
E. Titration of an Acid with a Base The base buret was washed thoroughly with soap and water and was rinsed with 3 mL portions of the standard base, 0.1 M NaOH. The buret was filled with 0.1 M sodium hydroxide, NaOH and the air space was removed at the tip. Zero reading was set and the buret was clamped into the iron stand. 10 mL aliquot of the unknown acid was transferred into an Erlenmeyer flask. 50 mL of distilled water and 2-3 drops of phenolphthalein was added. The acid was titrated with the standard base until the appearance of the first appearance of a permanent light pink coloration. The final reading was recorded and three trials were conducted. Finally, the molarity of the unknown acid solution was calculated.
REAGENTS 0.1 M SOLUTIONS
CONDUCTIVITY (GOOD/WEAK/NONE)
a. NaOH b. NH4Cl c. HCl d. HC2H3O2 e. NaCl H2O f. Sucrose
Good Good Good Weak Good None None
G. Ethanol HOAc + NaOH
None ---
HCl + NaOH
---
Table 2. Conductivity
B. Preparation of 1 M NaOH using NaOH pellets Calculations:
V. Results A. Electrolytes In the observation of the effects of the addition of the indicators (litmus paper, phenolphthalein, congo red) in the given solutions, the following results were obtained: REAGENTS 0.1 M SOLUTIONS a. NaOH b. NH4Cl c. HCl d. HC2H3O2 e. NaCl H2O f. Sucrose G. Ethanol HOAc + NaOH HCl + NaOH
CLASSIFICATION (ACIDS, BASES OR NEUTRAL) Base Neutral Acid Acid Neutral Neutral Neutral Base Acid Base
pH 12 7 1 3 7 7 7 4 4 13
1 M NaOH = Xg NaOH 40g NaOH 100g x 1 mL x 1 L 1g 100 mL 0.1 = x 40 x = 4g NaOH Pellets C. Preparation of 0.1 M NaOH from available concentration of NaOH Calculations: (1 M)(x) = (0.1 M NaOH final) (100 mL NaOH + H20 Final) x= 10 mL NaOH initial D. Titration of an Acid with a base
Table 1. Classification of Solutions and pH Level
Using the litmus paper, acids turned the blue litmus into red while the bases turned the red litmus into blue. The neutral solutions did not change the color of the litmus.
Chem 14.1, Acids, Bases and Salts
Volume of 0.10 M NaOH
TRIAL No. 1 9.0 mL
No. 2 8.5 mL
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Volume unknown solution
of acid
10 mL
10 mL
Molarity unknown solution
of acid
0.900 M
0.850 M
Table 3. Titration Results
Average Molarity of unknown acid solution: 0. 0875 M Calculations: MA = MBVB VA MA = (0.1M) (0.0875L) = 0.0875 M (0.1L) VI. Discussion A. Electrolytes Acid is a substance that produces H+(aq) ions in aqueous solutions. Strong acids ionize completely or almost completely in dilute aqueous solutions; weak acids ionize only slowly. Base is a substance that produces OH-(aq) ions in aqueous solutions. Strong bases are soluble in water and are dissociated completely. Weak bases ionize only slowly. Salt is a compound that contains a cation other than H+ and an anion other than OH- or O2-. Aqueous solution of both acids and base are electrolytes since they conduct electricity due to their ionization ability. The indicators used in the experiment were litmus paper, pH paper, phenolphthalein, and congo red. Indicators are weak acids or bases with differently colored acid and base forms. Chemical reaction is responsible for the indicator action in which change in color occurred in the experiment. The indicator reaction is pH dependent because it involves either the release or capture of hydrogen ions: HIn H+ + Inwhere "HIn" and "In" stand for the indicator molecule with and without an attached hydrogen ion. The two forms of the indicator molecule have noticeably different colors. For example, phenolphthalein has a clear HIn form and a redviolet In form. When there are equal amounts of HIn and In, the solution looks pink/cloudy. Adding a Chem 14.1, Acids, Bases and Salts
drop of acid adds H+ ions which react with the Inions to form HIn, and the solution becomes more clear. Adding a drop of base converts HIn to In, and the solution becomes more pink/red violet. Litmus is a weak acid. It has a seriously complicated molecule which we will simplify to HLit. The "H" is the proton which can be given away to something else. The "Lit" is the rest of the weak acid molecule. There will be an equilibrium established when this acid dissolves in water. Taking the simplified version of this equilibrium: Phenolphthalein is another commonly used indicator for titrations, and is another weak acid.
In this case, the weak acid is colorless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colorless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink.
Approximate pH range for color change: 8.0-9.8 Color of acid form: clear Color of base form: red-violet Congo Red
Approximate pH range for color change: 3.0-5.0 Color of acid form: blue Color of base form: red For part A electrolytes, the different reagents were tested using the following instruments: phenolphthalein, congo red, pH paper, litmus paper and conductivity apparatus. Through these indicators, properties like acidity, conductivity, the strength and weakness of acid/ base and the electrolyte property were obtained. For 0.1 M sodium hydroxide, the red litmus paper turned blue, and the blue litmus paper turned red, indicating that the reagent is basic. Its pH is 14, and through the conductivity apparatus, it was identified that it is a strong Electrolytes are compounds that ionize or dissociate into their constituent ions to produce aqueous solutions that conduct an electric current. Strong electrolytes are ionized or dissociated completely or very nearly completely, in
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dilute aqueous solutions. Strong electrolytes include strong acids, strong bases and strong salts. Weak electrolytes conduct electricity poorly in dilute aqueous solutions. Non-electrolytes exist as molecules in aqueous solutions, and such solutions do not conduct electric current. Electric current is carried through aqueous solution by the movement of ions. The strength of an electrolyte depends on the number of ions in solution and also on the charges of these ions. pH is a measure of the acidity or basicity of a solution. It is defined as the negative logarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations. The pH scale is not an absolute scale; it is relative to a set of standard solutions whose pH is established by international agreement. VII. Guide Questions & Answers 1. From your results, what relationship can you draw between the acidity/basicity of a substance and its electrolyte property? The strength of the conductivity of the electrolyte is determined on the strength of its acidity/basicity. The stronger acidity/basicity, the stronger the electrolyte while the weaker the acidity/basicity, the weaker the electrolyte. For neutral solutions, due to its ionic property, it also conducts electricity and is therefore considered an electrolyte. 2. Why are electrolytes conductors of electricity? Electrolytes can conduct electricity because of their ability to ionize in water. The electrodes attract the ions making a movement that sets up an electric current, thus generating electricity. 3. Calculate the pH of the following solutions: a. 0.001 M HCl [H+] = 0.001 M pH = -log(0.001M) pH = 3 b. 0.005 M NaOH [OH-] = 0.005 M pOH = -log(0.005M) pOH = 2.30 pH = 14 – 2.30 pH = 11.70 Chem 14.1, Acids, Bases and Salts
c. 0.10 M NH4OH solution (Kb = 1.8x10-5) Kb = [NH4+] [OH-] = 1.8x10-5 [NH4OH] Kb = x2 = x2 = 1.8x10-5 0.10-x 0.10 x2 = 7.2x10-6 x = 2.7x10-3 pOH = -log(2.7x10-3) = 2.57 pH = 14 – 2.57 = 11.43 d. 0.2 M HCOOH (Ka = 1.8x10-4) Ka = [COOH-] [H+] = 1.8x10-4 [HCOOH] Ka = x2 = 1.8x10-5 0.2-x 0 = x2 + x(1.8x10-5) – 0.2(1.8x10-5) x = -6.09x10-4 and/or 5.91x10-4 pH = -log(5.91x10-4) pH = 3.23 VIII. Conclusion and Recommendations In this experiment, we found out that litmus paper, phenolphthalein, congo red and pH papers may be used to determine whether substances are acids, bases or salts and a conductivity apparatus can determine whether a solution is an electrolyte or not. In order to know the concentration of an unknown solution, determining the endpoint of an acid-base titration can be used. IX. References Chang, R. (2002). Chemistry. Singapore. McGraw Hill Inc.
7th
Edition.
(n.a).(n.d) Retrieved from http://www.scribd.com/doc/30014852/Expt7-Acids-Bases-and-Salts, Feb. 28, 2011. I hereby certify that I have given substantial contributions to this report. ----------------------------------Espiritu, Adelaine Joy F. ----------------------------------Galvan, Ma. Danica Katrina P. Page 4 of 4
