A student isolates a colorless crystalline solid that she believes to be pure benzoic…
CHEMISTRY · Multi-part · A Level · Chemistry · Organic analysis and spectroscopy · Organic analysis and spectroscopy
Problem
A student isolates a colorless crystalline solid that she believes to be pure benzoic acid (melting point 122.4 °C). When she determines the melting behavior, the sample begins to melt at 112 °C and is completely liquid at 118 °C. She therefore concludes that the substance must be a mixture.
An infrared (IR) spectrum of the solid shows only the characteristic absorptions of benzoic acid, and a combustion analysis gives exactly the theoretical percent composition of benzoic acid (C₇H₆O₂).
Answer the following questions to evaluate the student’s conclusion.
(a) Discuss whether the melting-point data alone are sufficient evidence to classify the sample as a mixture. Include in your discussion the significance of the melting-point range for assessing purity.
(b) Propose two additional experimental tests—other than IR spectroscopy and combustion analysis—that could help determine the purity of the benzoic acid sample. For each test, briefly describe the expected observations for a pure substance versus a mixture.
(c) If all available evidence (including the additional tests proposed in part (b)) supports the conclusion that only benzoic acid is present, explain at least one plausible reason—unrelated to chemical impurity—why the melting point of the sample is depressed and broadened relative to the literature value.
(d) Based on the collective evidence, state whether the original solid should ultimately be classified as a pure substance or a mixture, and provide a comprehensive justification for your decision.