Acids and Bases

Topics: PH, Acid dissociation constant, Acid Pages: 7 (2330 words) Published: June 20, 2013
Standardization of NaOH and Determination of Unknown Acid

KEYWORDS: quantitative analysis, titration, buret, endpoint, standardization, half-equivalence point, calorimetric titration, potentiometric titration
ABSTRACT: The concentration of sodium hydroxide was determined by colorimetric titration, and the identity of an unknown acid was determined by potentiometric titration. In the first titration, a strong acid standard, potassium hydrogen phthalate (KHP), was used, to determine the concentration of a strong base, sodium hydroxide (NaOH). In order to do so, we prepared NaOH solution, prepared a buret, and standardized this solution by performing a colorimetric titration of the KHP with the solution until a color change was present. The color change was introduced by an indicator known as phenolphthalein, which caused the solution to go from colorless to pink, which marked the endpoint of this titration. As a result, the concentration of the NaOH solution was found to be 0.124 M with a standard deviation of 0.004 M. In the second titration, the goal was to use both an acid’s formula weight and acid dissociation equilibrium constant (pKa) value to determine the identity of this unknown acid. Both values were found by preparing the unknown acid solution, preparing a buret, titrating the acid solution with the strong basic NaOH solution from the previous experiment, and determining the pH at various points using the pH meter. The formula weight of our unknown acid was found to be189.93 g and the pKa was 7.2 and 7.05 using two different methods. We were able to conclude that our proposed identity of our unknown acid was MOPS, which had a formula weight of 209.26 g and pKa of 7.2.

Introduction
Concepts
Through both the standardization of sodium hydroxide and determining unknown acids, the titration process plays a major role. Titration is one of the types of quantitative analysis, which refers to the determination of how much a given component is present in the sample and is then used to determine unknown concentration of various chemicals. It is a very delicate and precise procedure that requires patience and specific observation by which the solution of a specific reactant is judiciously added to another reactant. In order for some solutions to be accurately prepared by mass, it is sometimes necessary to measure the concentration of a diluted solution by using it to titrate to a known amount of acid, which is known as standardization. The concentration of sodium hydroxide is standardized using potassium hydrogen phthalate (KHP). An indicator, which is a weak acid or weak base, is used in titration to determine the endpoint of the reaction as it changes the color of the solution, which is then used to determine the concentration of the unknown. The endpoint occurs when the moles of acid is equivalent to the moles of base. The indicator for the first titration was phenolphthalein. The colorless phenolphthalein is the protonated form, which is the conjugate acid, and the pink phenolphthalein is the deprotonated form, which is the conjugate base. Chemical Equations

The chemical equation that acts as an acid and base reaction is:
NaOH(aq) + HCl NaCl(aq) + H2O(l) (1)
Acids are seen as substances that contained hydrogen and bases as substances that contained hydroxide. In a reaction between an acid and a base, hydrogen ions from the acid react with hydrogen ions from the base forming water.

The chemical equation that represents the ionization of a weak acid is
HA + H2O H3O+ + A-(2)
which is also known as a Brønstead-Lowry acid-base interaction. In this reaction, HA represents the free acid, H3O+ and A- represent its dissociation products, and H2O acts as the base. Mathematical Equations

Once one has completed the titration, it is then possible to use the mass of the acid, molecular weight of the base, and the volume of the base to determine the concentration of the base, which is represented by...

References: (1) Neuman, R.C.; Kauzmann, W.; Zipp, A. The Journal of Physical Chemistry. 1973, 77, 2687-2691.
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