Identifying an Unknown Weak Acid

Identifying an Unknown Weak Acid - Identifying an Unknown...

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Identifying an Unknown Weak Acid Hunter Morgan, Katelyn Heck, and Jamie Elise King Chemistry 1212 Lab Matthew Morgan November 26, 2007
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Introduction The acidity of a solution is a very important aspect of life when it comes to food, water, and agriculture among other things. The strength and quality of acid is a very important aspect of our daily lives. The strength of an acid must be measured in order to find the quality of these solutions with respect to their purpose in our lives. One way to measure the strength of an acid is through the acid ionization constant, K a . This is the quantitative measure of the strength of the acid through its ability to donate protons to a base. The following equation of the ionization of a generic acid, HA, can be used to find the corresponding acid ionization constant expression: [H 3 O + ][A - ] Ka= ---------------- [HA] The corresponding acid ionization constant expression, K a , is a characteristic of an acid and is often used to identify an unknown acid. The K a constant represents the strength of the acid; thus, a stronger the acid has a larger the K a value, and a weaker the acid has a lower K a value. In this experiment, we will determine an unknown, monoprotic acid solution by using three experimental techniques. The first technique we will use to determine the unknown acid is titration of the acid with 0.100 M NaOH. We will titrate 80 mL of the 0.100 m unknown acid solution with sodium hydroxide, which will produce a titration curve plotting the pH of the acidic solution versus the volume of sodium hydroxide added. Once all of the unknown acid has completely reacted with the sodium hydroxide, the equivalence point has been reached. The equivalence point is the center region of the titration curve in which the pH sharply increases. The charted equivalence point is used
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to find the half-equivalence point, which is the point in the titration that exactly one half of the neutralization of the acid has occurred, which also shows half of the needed base has been used. At the half-equivalence point, the concentration of the unknown acid in the solution, [HA] is equal to the neutralizing base [A - ]. Which is written as: [HA]=[A - ] This allows the first equation to be simplified into: K a =[H 3 O + ] The negative logarithm of each side will allow us to derive an equation to determine the uknown acid’s pH: -log(K a )= -log[H 3 O + ] pK a = pH The pK a for the acid is equal to the pH of the acid at the half-equivalency point. The following equation is used to determine the K a of the acid: K a =10 -pK a The second experimental technique we will use is determining the percent ionization of the acid from the freezing point of depression. A curve will be produced plotting the freezing point of the acid which will allow us to determine the unknown solution based on its freezing point relative to water. Once the solution freezes, the freezing point of depression of the unknown acid will allow us to identify the acid.
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