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The Use of Volumetric Flask, Burette and Pipette in Determining the Concentration of NaOH Solution

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Introduction

Title The Use of Volumetric Flask, Burette and Pipette in Determining the Concentration of NaOH Solution Objective * To determine the number of ionizable hydrogen in an unknown acid. * To determine the equivalent weight of an unknown acid. * To determine the enthalpy change for the ionization of an unknown acid. * To use the technique of volumetric analysis or titration to determine the concentration of a given NaOH solution. Theory And Background In 1855, the German chemist, Friedrich Mohrn defined titration as the "weighing without scale" method because this process allows determination of the concentration of a sample without using complex instrumentation. A manual titration requires high accuracy and precision, both in the preparation of the material, and the use of different precisely dosed reagents. The operation must be repeated at least 3 times to obtain a reliable measured value. This procedure makes the manual analytical technique very long and fastidious. Titration is the quantitative measurement of an analyte in solution by reacting it completely with a standardized reagent. For example, a given volume of a solution of unknown acidity may be titrated with a base of known concentration until complete Neutralization has occurred. Acids and bases react until one of the reactants is consumed completely. A solution of base of known concentration can therefore be used to titrate an acid solution of unknown concentration. Likewise, an acid solution of known concentration can be used to titrate a base solution of unknown concentration. The point at which all of the analyte is consumed is the equivalence point and is generally determined by observing a color change in an added indicator such as phenolphthalein. The term "end point" is where the indicator changes colour. That isn't necessarily exactly the same as the equivalence point. This means that at the equivalence point (where you had mixed the solutions in the correct proportions according to the equation), the solution wouldn't actually be neutral. ...read more.

Middle

The process of determining the concentration of a solution is called standardize. The point at which all of the analyte is consumed is the equivalence point. The number of moles of analyte is calculated from the volume of reagent that is required to react with all of the analyte, the titrant concentration, and the reaction stoichiometry. Indicators that are available for titrations based on acid-base neutralization, complextion, and redox reactions often determine the equivalence point. For acid-base titrations, indicators are available that change color when the pH changes. When all of the analyte is neutralized, further addition of the titrant causes the pH of the solution to change causing the color of the indicator to change. A suitable indicator added to the unknown solution changes colour when the stoichiometric required amount of standard solution is added. Standard solution is added drop wise until a single drop causes the indicator undergo permanent colour change. This is called the end point. Phenolphthalein is a sensitive pH indicator with the formula C20H14O4. It is often use in titration, its turns from colourless in acidic solutions to pink in basic solutions, the colour change occurring between pH 8 and pH 10. If the concentration of indicator is particularly strongly, it can appear purple. All acid-base titration reactions are simply exchanges of protons. The reaction may be strong acid + strong base --> salt (neutral). For this experiments, we use hydrochloric acid and sodium hydroxide, the equation are as below: HCl + NaOH --> NaCl + H2O Although the reaction may be correctly written as H3O+ + OH- --> H2O since strong acids and strong bases are totally dissociated to protons and hydroxide ions in water. For reactions which are strong acid + weak base --> salt (acidic). For example HCl + CH3NH2 --> CH3NH3+Cl-, For reaction which are strong base + weak acid --> salt (basic) For example NaOH + CH3COOH --> Na+CH3COO- + H2O The cations and anions could be omitted as they do not actually participate in the reaction. ...read more.

Conclusion

The possible error in this experiment were: the error in taking the burette readings, the error in measuring amount of elements, and the NaOH was not stable under air. Questions: (1) Calculate the concentration of NaOH solution. Concentration of base (diluted solution) M1V1 = M2V2 M1(12.40) = (0.01)(25) M = (0.01)(25)/12.40 = 0.0202M The concentration of the diluted NaOH is 0.0202 molar, which is approximately 0.02 molar. From the molar of the diluted NaOH, we compare the concentration of the original NaOH, Concentration of base (original solution) M1V1 = M2V2 M1(50) = (0.0202)(250) M = (0.0202)(250)/50 = 0.0101M The concentration of the NaOH solution used is 0.0101 molar. (2) Distinguish between acid strength and acid concentration. Acid strength is the percentage of ionization of the acid when dissolve in water while acid concentration is the amount of dissolved acidic solutes in the solution. (3) Distinguish between a weak base and an insoluble base. A weak base is a chemical base that does not ionize fully in an aqueous solution. This results in a relatively low pH level. Weak bases exist in equillibrium much in the same way as weak acids do, with a Base Ionization Constant (Kb) indicating the strength of the base. Not many metal hydroxides are soluble; the ones that are comprise the strong soluble bases. Hydroxides that are only slightly soluble in water (such as calcium hydroxide or iron(III) hydroxide) are strong bases, because whatever amount does dissolve dissociates completely into the ions. So, we can say that weak base has a lower pH level compared to insoluble base because weak base does not ionize fully in aqueous solution, whereas insoluble base are strong base because most of them ionize fully in water. Conclusions From the titration results of three trials, the concentration of NaOH solution in the diluted acid solution is 0.0202M. The concentration of NaOH solution in the original acid solution is 0.0101M. ...read more.

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