CHEM 235 Study Guide - Quiz Guide: Diethyl Ether, Biphenyl, Sodium Hydroxide

1. What would be the effect on the observed melting point if sample were: a) too small b) too large- c) poorly packed d) heated too rapidly- 2. Some compounds sublime in the capillary and some decompose before melting. How do you determine melting point of these compounds? 3. A student was given a white solid for an unknown. Its melting point range was 119 121 °C. The student has previously worked with benzoic acid, and had observed that it was a white crystalline solid with a melting point of 122 °C. (a) Can the student conclude that the unknown is benzoic acid on the basis of her work to this? Why or why not? (b) What additional experimental work should be done to verify this compound? 4. You and your lab partner take melting points of the same sample. You observe a melting point of 101-107°C, while your partner observes a value of 110-112°C. Explain how you can get two different values with exactly the same sample. 5. An unidentified compound is observed to melt sharply at 111 °C with the vigorous evolution of a gas. The sample then solidifies and does not melt until the temperature reaches 155 °C, at which time it again melts sharply. Briefly explain these observations.

1. What would be the effect on the observed melting point if sample were: a) too smal b) too large- c) poorly packed d) heated too rapidly- Some compounds sublime in the capillary and some decompose before melting. How do you 2. determine melting point of these compounds? 3, A student was given a white solid for an unknown. Its melting point range was l 19-121 ℃ The student has previously worked with benzoic acid, and had observed that it was a white crystalline solid with a melting point of 122 ℃ (a) Can the student conclude that the unknown is benzoic acid on the basis of her work to this? Why or why not? (b) What additional experimental work should be done to verify this compound? You and your lab partner take melting points of the same sample. You observe a melting point of 101-1070C, while your partner observes a value of i 10-112°C Explain how you can get two different values with exactly the same sample. 4. 5. An unidentified compound is observed to melt sharply at 1 1 1 °C with the vigorous evolution of a gas. The sample then solidifies and does not melt until the temperature reaches 155 °C, at which time it again melts sharply. Briefly explain these observations.

What effect would poor circulation of the melting point bath liquid have on the observed melting point? What is the effect of an insoluble impurity, such as sodium sulfate, on the observed melting point of a compound? Three test tubes, labeled A, B, and C, contain substances with approximately the same melting points. How could you prove that the tubes contain three different chemical compounds? One of the most common causes of inaccurate melting points is too rapid heating of the melting point compare to the true melting point? Strictly speaking, why is it incorrect to speak of a melting point? What effect would the incomplete drying of a sample (for example, the incomplete removal of a recrystallization solvent) have on the melting point? Why should the melting point sample be finely powdered? You suspect that an unknown is acetanilide (mp 113.5 degree C-114 degree C). Give a qualitative estimation of the melting point when the acetanilide is mixed with 10% by weight of naphthalene. You have an unknown with an observed melting point of 90 degree C-93 degree C. Is your unknown compound A with a reported melting point of 95.5 degree C-96 degree C, or compound B with a reported melting point of 90.5 degree C-91 degree C? Explain. Why is it important to heat the melting point bath or block slowly and steadily when the temperature gets close to the melting point? Why is it important to pack the sample tightly in the melting point capillary? An unknown compound is suspected to be acetanilide (mp 113 5 degree C-114 degree C). What would happen to the melting point if this unknown were mixed with (a) an equal quantity of pure acetanilide? (b) an equal quantity of benzonic acid? Which would be expected to have the higher boiling point t-butyl alcohol (2-methyl-2-proponal) or n-butyl alcohol (1-butanol)? Explain. What is the purpose of the side arm of the thermometer adapter in Figure 3.13?