CHE 169 Exp. #6 THE VOLUMETRIC ANALYSIS OF ACIDS AND BASES 1. introduction The p
ID: 1032965 • Letter: C
Question
CHE 169 Exp. #6 THE VOLUMETRIC ANALYSIS OF ACIDS AND BASES 1. introduction The process of determining the amount of a substance present by measuring the volume of a solution of a second substance that reacts with it in some definite and complete fashion is known as volumetric analysis. The solution of the second substance must be of known concentration and is referred to as a standard solution. The process of carrying out the reaction by, in many instances, adding the standard solution from a buret to a flask containing a solution of the unknown substance, is known as a titration. Volumetric analysis is also referred to as titrimetric analysis It is essential in any volumetric analysis that some method be available for detecting when just the sufficient amount of the standard solution has been added to react completely with the substance being determined; that is, some method must be available for finding the equivalence point or the point corresponding to the mixing of exactly the necessary amounts of the two substances involved in the chemical reaction. The most common method of doing this is be estimating the equivalence point by the addition of another substance, called an indicator, which changes color at or very near the equivalence point of the reaction. This estimated point is known as the end-point. For accurate work, it is necessary for the equivalence point and the end-point to be very nearly identical. A great many natural and synthetic organic materials are known, which change color with a change in acidity, and can serve as indicators in titrations involving acids and bases. However, not all such substances are equally suitable for all acid-base combinations. The indicator phenolphthalein, which is pink in moderately to strongly basic solutions and colorless in acidic, neutral and mildly basic solutions, is often used in such titrations. Phenolphthalein is suitable for end-point detection in the experiments to follow. The end-point obtained is very sharp; only a drop, or sometimes only a fraction of a drop, will bring about a distinct color change. The process of volumetric analysis is in a very real sense a comparison process: comparing a solution of unknown concentration with a solution of known concentration. The concentration of the titration with a solution of known concentration. This process is frequently referred to as a standardization. The concentration of the standard solution to be used is usually determined by the titration of the standard solution and a solution of a pure solid material called a primary standard. A primary standard is a highly pure substance that can be weighed and made into a solution of known concentration by carefully dissolving it in a given quantity of solvent. Ideally, a primary standard should have a high molar mass and a composition that is chemically and physically stable. In the work to follow, oxalic acid dihydrate is used as the primary standard in the standardization of a sodium hydroxide solution. Once the concentration of the sodium hydroxide is determined, it too becomes a standard solution and may be used to determine the concentration of any number of acid solutionsExplanation / Answer
1. The molar mass of H2C2O4.2H2O in g/mol
2C = 2*12.011 = 24.022
6H = 6*1.008 = 6.048
6O = 6*15.999 = 95.994
Therefore, the molar mass of H2C2O4.2H2O = 24.022 + 6.048 + 95.994 = 126.064 g/mol
2. (a) The no. of millimoles of H2C2O4.2H2O in 250 mL of 0.05 M acid solution = 250 mL * 0.05 mmol/mL = 12.5 mmol
The mass of H2C2O4.2H2O = 12.5*10-3 mol * 126.064 g/mol = 1.5758 g (since 1 mol = 103 mmol)
(b) The no. of moles of oxalic acid contained in 12 mL of the solution in part A = 12 mL * 0.05 mmol/mL
= 0.6 mmol
= 6*10-4 mol