Acid-Base Titration Chemistry Formal Lab Writeup by A.Mm

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Abstract

By using acid-base titration, we determined the suitability of phenolphthalein and methyl red as acid base indicators. We found that the equivalence point of the titration of hydrochloric acid with sodium hydroxide was not within the ph range of phenolphthalein's color range. The titration of acetic acid with sodium hydroxide resulted in an equivalence point out of the range of methyl red. And the titration of ammonia with hydrochloric acid had an equivalence point that was also out of the range of phenolphthalein.. The methyl red indicator and the phenolphthalein indicator were unsuitable because their pH ranges for their color changes did not cover the equivalence points of the trials in which they were used. However, the methyl red indicator is more suitable, since it's pH range is closer to the equivalence points of the titrations.

Introduction

Acid-base reactions are one of the most common and important chemical interactions. They are vital to both environmental and industrial systems. As an important variable, pH controls the toxicity, mobility, solubility, and fate of many aquatic ecosystems. Most aquatic life forms cannot survive outside a pH window from about 4.5 to 9. From an industrial viewpoint, manipulation of pH is both a tool for and a prerequisite to all water treatment processes.1 Along with pH indicators, titration is a vital tool in determining the factors of many commercial and environmental systems.

Therefore, knowledge of acid-base titration curves is critical to the environmental scientist. Titration, an analytical technique, allows the quantitative determination of a dissolved substance being titrated, known as an analyte. Titration requires knowledge of the equivalence point: a theoretical point where the chemical equivalents of titrant added are exactly equal to the chemical equivalents of the solute being titrated. It also requires the knowledge of the Endpoint: an operational point which approximates the position of the equivalence point by some physical change.1

The purpose of this lab is to find the most suitable indicator. In the lab we used, Phenolphthalein or Methyl-Red, for differing titrations. Here are some other indicators: 1

In this lab we used the following reactions and equations in this lab:

Reaction 1: The neutralization reaction of Sodium Hydroxide and Hydrochloric Acid:

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

Reaction 2: The complete dissociation reaction of Hydrochloric Acid:

HCl (aq) + H2O (l) ?H3O+ (aq) + Cl- (aq)

Reaction 3: The incomplete dissociation reaction of Acetic Acid:

CH3COOH (aq) + H2O (l) ? H3O+ (aq) + CH3COO- (aq)

Reaction 4: The reaction of the Acetate Ion with Water:

CH3COO- (aq) + H2O (l) ?CH3COOH (aq) + H3O+ (aq)

Reaction 5: The reaction of Ammonia with Hydrochloric Acid:

NH3 (aq) + HCL (aq) ?NH4+ (aq) + Cl- (aq)

Reaction 6: The reaction of the left over Ammonia with Water:

NH4+ (aq) + H2O (l) ?NH3 (aq) + H3O+ (aq)

The three following mathematical equations were used :

Equation 1: To calculate the mass of a substance needed to prepare a solution:

Mass of substance ( in grams) = (M)(V)(Formula Weight)

Equation 2: To calculate the pH:

pH = -log [ H3O+]

Equation 3: To calculate the volume of titrant added to the analyte to produce a final solution with a specific Hydronium concentration:

[ H3O+] = (Vanalyte x Manalyte ) - (Vtitrant)(Mtitrant) / (Vtitrant + Vanalyte)

Equation 4: To calculate the number of moles:

Number of Moles = Molar Concentration Volume = Mass Molar Mass

Experimental

Materials Apparatus

-0.1M NaOH (prepared from solid NaOH)

-0.1M CH3COOH (aq)

-0.1M Hcl (aq)

-0.1M NH3 (aq)

-Phenolphthalein indicator

-Methyl-red indicator (1 (1

-electronic pH meter

-hot plate/stirrer

-50mL burette with stand

-250mL beaker

-100mL beaker

-25mL volumetric pipette

-pipette safety bulb

-scoopula

-magnetic stir bar

-plastic...
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