# Composition of Impure Weak Acid. 1.docx - Composition of...

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Composition of Impure Acid Dates Performed: October 22, 2018 and October 29, 2018 Date Due: November 5, 2018
Abstract. The goal of this experiment is to prepare an aqueous solution of sodium hydroxide and determine its concentration by titrating it with a solution containing a precise mass of potassium hydrogen phthalate KHC 8 H 4 O 4 (abbreviated KHP). The chemical reaction that occurs in this experiment is NaOH ( aq ) + KC 8 HO 4 H ( aq ) → NaKC 8 HO 4 ( aq ) + H 2 O ( l ). Through this equation, it is clear to ntoe that the mole ratio between KHP and Introduction. When completing titrations, it is important to know the molarity of the titrant. In this lab, a titrant that is strongly hygroscopic, meaning that it strongly absorbs water from the air, and this means that NaOH cannot be weighed accurately, and thus, only an approximate concentration can be taken. A common way that is used to standardize the NaOH solution is by using the weak acid known as, potassium hydrogen phthalate (KC 8 HO 4 H or KHP). An accurate mass of KHP can be taken, using mass by difference, and be titrated with the NaOH determine the concentration of NaOH. When this acid- base reaction occurs the chemical equation it yields reads: NaOH ( aq ) + KC 8 HO 4 H ( aq ) → NaKC 8 HO 4 ( aq ) + H 2 O ( l ). The initial pH value of the KHP is acidic, but after the titration occurs, it will become a very basic solution. The equivalence point of the solution will be greater than the neutral pH. In order to determine the exact pH of the solution that is being titrated, a pH meter must be used. This measures the activity hydrogen-ion activity in the solution. In order to get an accurate reading on the pH meter, it must first be calibrated using pH buffers 4, 7 and 10. The method that is used to determine the molarity of your NaOH solution is called a colorimetric titration, in which phenolphthalein is the indicator used. Phenolphthalein is an acid-base indicator that is colorless in acidic solutions and pink in basic solutions, especially those that have an excess of base. The acid base indicator is a compound whose color depends on the acidity of a solution. This process used to determine if and when the neutralization reaction is complete. Students have to make sure the light pink color persists for 30 seconds. For phenolphthalein, if the pH is less than 8, it is colorless, but when it is higher than 8, it is a pink color.
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