Final answer:
A back titration is used instead of a direct titration to determine calcium carbonate content in eggshell due to impurities and the insolubility of calcium carbonate, allowing for accurate quantitative chemical analysis despite interfering substances.
Step-by-step explanation:
The student performed a back titration rather than a direct titration to determine the percentage of calcium carbonate in eggshell primarily because the eggshell may contain impurities that do not react in a predictable manner with a direct titration. Back titrations are useful when the substance being analyzed is insoluble in water, as is the case with calcium carbonate in eggshells, or when it contains interfering substances that can affect the reaction. With back titration, an excess of standard acid is reacted with the calcium carbonate, and then the remaining unreacted acid is titrated. This allows for the calculation of the amount of acid that reacted with the calcium carbonate and, hence, the amount of calcium carbonate in the sample.
Quantitative chemical analysis using titrations relies on the stoichiometry of chemical reactions, where one can calculate the concentration of analyte in the sample based on the volume of titrant used. Back titration is a form of indirect analysis which is sometimes necessary when a direct titration cannot provide a clear endpoint or when the sample has properties that make direct titration unsuitable.