Experiment No. 7 ACIDS, BASES AND SALTS

Ferrer, Abigael G.Fong, Michael

Group 6, Chem 14.1, FAB2, Prof. Mario T. Rosete IIIMarch 19, 2010

Experiment 7 is divided into five parts. The first and second part is the classification of the electrolytes and the conductivity test, respectively. The third part consists of the preparation of 1 M NaOH using NaOH pellets and the fourth part consists of the preparation of 0.1 M NaOH from the available concentration of NaOH. The last part is the titration of an acid with a base, specifically an unknown acid with a .1 M NaOH base.

Keywords: acid, base, salt, electrolyte (strong and weak), non-electrolyte, conductor (good and weak), non-conductor, pH, titration, equivalence point, end point

Introduction

The experiment aims to explain and apply concepts regarding electrolytes and titration.

An electrolyte is any substance containing free ions that make the substance electrically conductive. They commonly exist as solutions of acids, bases or salts. Acids, bases and salts have many uses and are important in different fields, not only in Chemistry.

Acids are often used to remove rust and other corrosion from metals. They may be used as an electrolyte in a wet cell battery, such as sulfuric acid in a car battery. Strong acids, sulphuric acid in particular, are widely used in mineral processing (ie phosphate minerals react with sulfuric acid to produce phosphoric acid for the production of phosphate fertilizers, zinc is produced by dissolving zinc oxide into sulfuric acid). They also react in neutralization reactions to produce salts and are used as additives to drinks and foods, as they alter their taste and serve as preservatives. Both acids and bases are used as catalysts. Salts, on the other hand, elicit all five basic tastes, e.g. salty (sodium chloride), sweet (lead diacetate; which will cause lead poisoning if ingested), sour (potassium bitartrate), bitter (magnesium sulfate), and umami or savory (monosodium glutamate). Salts are also used in the formation of crystals.

Titration, another concept in this experiment, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant. It is a process wherein a solution of accurately known concentration, called a standard solution (in this experiment,

NaOH-), is added gradually to another solution of unknown concentration, until the chemical reaction between the two solutions is complete.

Experimental

Part I. Electrolytes

5 drops of the following 0.1 M solutions (NaOH, NH4Cl, HCl, HC2H3O2, NaCl, C2H5OH, C12H22O11) with distilled water were first placed in different test tubes. They were then tested using litmus paper, phenolphthalein, and congo red. Color changes were noted and the substances were classified into acids, bases and salts according to these changes.

The pH levels were then determined using pH paper. To use the pH paper and the other indicators, distilled water was first placed in a watch glass. Then, the substance was added and finally, the indicator. This is in line with the Arrhenius definition wherein acids and bases are to be reacted with water.

Equal volumes (1mL each) of 1 M HCl and 1 M NaOH were mixed in a test tube and were tested with the available indicators. Then, this was repeated using 1 M acetic acid in place of HCl.

Part II. Conductivity Test

A small amount of substance to be tested was placed in a small beaker. The metal ends of the conductivity apparatus were placed inside this beaker. The light bulb was then observed. If it did not light up, the substance is not an electrolyte. If it lit up dimly, it was a weak electrolyte. If it lit up brightly, it was a strong

Chem 14.1 Acids, Bases and Salts Page 1 of 5

electrolyte. All substances (NaOH, NH4Cl, HCl, HC2H3O2, NaCl, C2H5OH, C12H22O11, HAc + NaOH, and HCl + NaOH) underwent the conductivity test.

Part III. Preparation of 1 M NaOH using NaOH pellets

The weight needed to prepare 100.00mL of 1 M NaOH were first calculated using the molarity (M) formula as shown below:

MM NaOH = 40.01 g/mol

M = Mass solute (MM)(L sol’n)

Mass solute = M (MM) (L sol’n) = 1.0 M (40.01 g/mol) (0.1L) = 4.0018 g

The calculated amount of pellets was weighed in a watch glass. Then, it was dissolved in 50mL water by swirling the flask. The solution (sol’n) was then transferred to 100-mL volumetric flask and then diluted by adding distilled water up to the mark in the volumetric flask.

Part IV. Preparation of 0.1 M NaOH from available concentration of NaOH

The volume of NaOH from available concentration (solution prepared in part III) needed to prepare 100mL of 0.1 M NaOH using the dilution formula (M1V1 = M2V2) as shown below:

MdiluteVdilute = MconcentratedVconcentrated

Vdilute = MconcentratedVconcentrated

Mdilute

= (0.1 M) (100mL) 1.0 M

= 10mL

The calculated volume was diluted to 100.00mL with distilled water in a volumetric flask.

Part V. Titration of an Acid with a Base

The base buret was first thoroughly washed with soap and water. Then, it was rinsed

three times with 3mL portions of the standard base (0.1 M NaOH). The buret was then filled with 0.1 M sodium hydroxide (NaOH) and airspace was removed. The reading was set to zero and finally, the buret was clamped to the iron stand.

A 10mL aliquot of an unknown acid (prepared by the instructor) was transferred into an Erlenmeyer flask. 50mL of distilled water and 2-3 drops of phenolphthalein were then added. The acid was titrated with the standard base until the first appearance of a permanent light pink coloration. The final volume reading was recorded. Three trials were made for accuracy. Then, the molarity of the unknown acid solution was calculated as shown below:

MacidVacid = MbaseVbase

Macid = MbaseVbase

Vacid

= (0.1 M)(.4 mL) 60 mL

= 6.67 x 10-4 MThe titration set-up is illustrated below:

Results

When blue litmus paper was used as an indicator, it turned red when tested on HCl and HC2H3O2 while it did not change for any of the other reagents. When red litmus paper was used, it turned blue when tested on NaOH while it did not change for any of the remaining reagents.

The addition of Phenolphthalein turned NaOH and NaOH with acetic acid pink whereas the addition of congo red turned NH4Cl, HCl, and HC2H3O2 blue.

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pH paper and the conductivity apparatus yielded the following results:

Reagents 0.1

M Solutio

ns

Classification (Acid,

base or neutral)

pH

Classification

of acid/ba

se (strong/weak)

Conductivity (good/weak)

Classification

(strong, weak, non-

electrolyte)

NaOH Base 13

Strong Good Strong

NH4Cl Acidic Salt

6 N/A Good Strong

HCl Acid 1 Strong Good StrongHC2H3

O2Acid 2 Weak Weak Weak

NaCl Neutral 7 Neutral Salt

Good Strong

H2O Neutral 7 N/A None Non-electrol

yteC12H22O11

(sucrose)

Neutral 7 N/A None Non-electrol

yte

CH3CH2OH(ethan

ol)

Neutral 7 N/A None Non-electrol

yte

HAc + NaOH

Basic Salt

8 N/A Good Strong

HCl + NaOH

Neutral 7 Neutral Salt

Good Strong

Calculations show that 4.0018 g of NaOH pellets are to be used in order to prepare 1M NaOH solution while 10 mL of 1M NaOH was needed to prepare 0.1 M NaOH through dilution.

According to calculations based on the titration results, the molarity of the unknown acid was 6.67 x 10-4 M.

Discussion

Acids and bases can be defined according to three concepts, the Arrhenius, Bronsted-Lowry and the Lewis concept. Acids produce H+ in H2O and bases produce OH- in H2O according to the Arrhenius concept. According to the Bronsted-Lowry definition, acids donate H+ while bases accept H+. The Lewis concept defines acids as e- pair acceptors while bases are e- pair donors.

In general, the litmus paper, phenolphthalein, and congo red tests were in line with the pH levels read using pH paper. pH

levels below 7 turned blue litmus paper red and reacted with congo red to form a blue colored solution. pH levels above 7, on the other hand, turned red litmus paper to blue and reacted to phenolphthalein to produce a solution that is colored light pink. It should be noted that litmus paper did not change color when reacted with either NH4Cl or HOAc + NaOH as both of these substances’ pH levels are too close to neutral for litmus paper to react. They did react accordingly with phenolphthalein and congo red, however.

NaOH reactions with HAc and HCl are: HAc + NaOH à H2O + NaAc HCl + NaOH à H2O + NaCl

As HAc is a weak acid, its conjugate base, NaAc, would be a strong base. NaOH, a strong base, had a weak conjugate acid in the form of water. Note that the reaction of HAc and NaOH formed a precipitate. This proves that the reaction created a salt. The reaction of HCl, a strong acid, and NaOH, a strong base, formed weak conjugate acids (water) and bases (NaCl). NaCl is a neutral salt whereas NaAc is a basic salt. This accounts for the difference in pH levels.

The conductivity apparatus was simply an open circuit which can only be a closed one if both metal end terminals were connected via another conductive material (in the experiment, an electrolyte-containing solution). Electrolytes are “any substance containing free ions that make the substance electrically conductive.” This is because ions, which have charges, allow electrons to move, allowing electricity to pass by. Strong electrolytes, therefore, would be good conductors of electricity while weak electrolytes would be weak conductors of electricity.

By definition, strong acids and strong bases completely dissociate into ions. It is precisely because of this reason that they are strong electrolytes. Conversely, weak acids and weak bases do not completely dissociate into ions so they do not create as much ions as strong acids and strong bases do – which is why they are weak electrolytes.

All salts, although not strong acids nor strong bases, are strong electrolytes and good conductors. This is because salts completely dissociate into ions just as strong acids and strong bases do.

The preparation of 1 M NaOH solution using NaOH pellets was an application of the definition of “molarity.” As molarity is the measure of the concentration of amount of solute per volume of solution, preparing 1 M NaOH solution simply required one to calculate

Chem 14.1 Acids, Bases and Salts Page 3 of 5

the required number of moles of the solute, NaOH, and the required volume of the solution (NaOH and water) and then to act accordingly by mixing them together. Initially, not all of the solvent was used. This is to make sure that all of the NaOH dissolves first before 1 M NaOH is created.

Dilution of 1M NaOH to 0.1 M NaOH was an application of the implications of the molarity formula. As molarity is expressed in mol solute over Liter solution, a mathematical viewpoint would reveal that molarity is inversely proportional to the volume of solution. Therefore, with the moles of solute being held constant, increasing the volume of the solution will decrease molarity. Therefore, 0.1 M NaOH was created from 1 M NaOH by adding more solvent (water) to the 1 M NaOH solution. Increasing the volume of the solution using a NaOH solution instead of water will not yield the desired molarity in the desired volume as using a NaOH solution will also increase the moles of NaOH instead of keeping it constant.

In the titration process, two points can be known, the equivalence point and end point. The equivalence point is the point at which the acid has completely reacted with or been neutralized by the base. At this point, no color change can be observed. On the other hand, end point is the point where the indicator (in this case, phenolphthalein) changes color due to the solution already becoming basic.

Ideally, titration calculations should be done at the equivalence point. However, detecting whether the solution has already reached this point is very hard to do. In practice, titration ends when the solution has reached the endpoint because this is easier to detect. The addition of excess titrant used to create color change would add a small positive error in the titration. This is why the end point is desired to be as close to the equivalent point as possible in order to decrease this error. As long as this error is in the same range as the burette precision ranges, the error can be ignored.

Conclusion and Recommendations

There are multiple ways to determine whether a substance is an acid, base, or salt. Using litmus paper, phenolphthalein, and congo read are some ways for one to test a substance. If blue litmus paper turned red or if the solution became blue due to congo red, the substance is acidic. If red litmus paper turned blue or if the solution became pink due to phenolphthalein,

the substance is basic. No changes on the litmus paper would indicate that a substance is neutral. Using pH paper can be used to determine pH levels of substances.

The conductivity test may be used to determine substances that are electrolytes. Those that lit up the light bulb are electrolytes while those that did not light up the bulb are non-electrolytes. By comparing the conductivity test results with that of the prior tests, one can find out if the electrolyte was a weak acid, a strong acid, a weak base, a strong base, or a salt. Dimly lit bulbs are weak acids or weak bases. Brightly lit bulbs are strong acids, strong bases, and salts.

Preparing solutions of certain molarities can be made through two different procedures. One is through dissolving the solute and combining enough solvent to come up with the desired concentration of solution. The other is through dilution wherein the concentration of a solution is lowered to the desired concentration by adding more solvent.

Titration is a process wherein the concentration of a known reactant can be determined. It is a process wherein a solution of accurately known concentration, called a standard solution, is added gradually to another solution of unknown concentration, until the chemical reaction between the two solutions is complete.

Preparation of solutions, and especially titration, require precise and accurate measurements of materials to be used. As such, conducting these experiments require as much trials as possible to lessen human error. For titration, patience should always be observed as doing the experiment drop by drop, although the most efficient and most accurate method, will take time.

References

(n.a) (n.d) Retrieved from http://en.wikipedia.org/wiki/electrolyte

(n.a) (n.d) Retrieved from http://en.wikipedia.org/wiki/acids

(n.a) (n.d) Retrieved from http://en.wikipedia.org/wiki/base

(n.a) (n.d) Retrieved from http://en.wikipedia.org/wiki/salt

(n.a) (n.d) Retrieved from

Chem 14.1 Acids, Bases and Salts Page 4 of 5

http://www.elmhurst.edu/~chm/vchembook/185strength.html

(n.a) (n.d) Retrieved from http://www.titrations.info/titration-basic-terms

Chang, R. (2007). Chemistry. New York: McGraw-Hill Companies, Inc.

I hereby certify that I have given substantial contribution to this report.

_______________________Ferrer, Abigael G.

______________________Fong, Michael

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