Determination of the Ionization Constant of a Weak Acid
February 12, February 19
& February 26, 2014
Abstract: Ordinary laboratory ware was used to determine the name of the unknown substance by using the pKa values of unknown acid which was 4.90 for trial 1 and 4.92 for trial 2 and mean molecular mass was 91.25 g/mol for trial 1 and 56.86 for trial 2. The name of the unknown acid was found to be Butyric.
Acid base reactions are one of the most common and familiar reactions involved in chemistry. When an acid is placed with a base they will react to neutralize the acid and the base properties, producing a salt. The H+ cations from the acid combine with the OH- anions from the base to form water. The compound formed by the cation of the base and the anion of the acid is called a salt. The word “salt” is a general term that applies to the products of an acid-base reaction. In essence, an essential feature of this "acid-base" process is the combination of hydronium ions with hydroxide ions, in aqueous solution, to form water, displayed by the chemical equation below:
One example of an acid-base reaction is the combustion of hydrochloric acid and sodium hydroxide which produces NaCl, common table salt.
The theory of acids and bases, like many other chemical theories, have undergone many changes in recent times. Many changes have been made to make the theory more general. There are three main theories are in use today. One of the theories is Water or Arrhenius Theory. The theory defines an acid as a hydrogen compound ionizing in water to give hydrogen ions, and a base a hydroxyl compound, which gives hydroxide ions in water. The neutralization reaction between an acid and a base produces salt and water only. The equation below represents the acid base reaction. The second theory is the Bronsted-Lowry Theory of acids and bases, the theory states that the acid is a proton (H+) donor and the base is the acceptor. The third theory is the Lewis theory the acid and base accept or donate an electron pair. The acid is the electron pair acceptor, whereas a base is an electron pair donor.
Acid-base titrations are useful in determining the concentration of an acid or base by neutralizing the acid/base exactly, with an acid or base of known concentration. The concentrations of the unknown acid or base solutions are to be determined quantitatively.
Acid-Base indicators are also known as pH indicators, which change color according to the pH. They can be bases or weak acids, when dissolved in water they disassociate from their ions slightly. Weak acid indicators are used with titrations that change under slightly alkaline conditions, whereas weak base indicators are used with titrations that change under slightly acidic conditions.
When the indicator changes the color of the solution then the end point of the titration has been reached. A color change normally will happen when two solutions are mixed together in exact equation proportions. That particular mixture is defined as the equivalence point. However, it remains a theoretical point, and can be estimated by observing a physical change associated with the condition of equivalence, such as a color change. The following rules are used to estimate the pH of the equivalence point: if a strong acid will react with a strong base, a neutral solution with a pH of 7 is formed; if a strong acid reacts with a weak base, an acidic solution with a pH greater than 7 is achieved; and similarly, if a weak acid reacts with a strong base, a basic solution with a pH value less than 7 is achieved. A weak acid is not normally titrated with a weak base because the color change of the indicator happens fairly quickly and is very hard to observe.
One common indicator used in titration is Phenolphthalein. It is another weak acid. In this case the weak acid is normally colorless but its ions have a light pink color. When adding more hydrogen ions to the…