Chapter 4 Melting Points, Boiling Points, and Refractive Indices Questions 1. Wh

Question

Chapter 4 Melting Points, Boiling Points, and Refractive Indices Questions 1. What effect would poor circulation of the melting point bath liquid have 63 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? 3 Three test tubes, labeled A, B, and C, contain substances with approxi- mately the same melting points. How could you prove the test tubes con- tain three different chemical compounds? 4. One of the most common causes of inaccurate melting points is too rapid heating of the melting point bath. Under these circumstances, how will the observed melting point compare to the true melting point? Strictly speaking, why is it speak of SJStrictly speaking, why is it incorect to speak of a melting point? 6 What effect would the incomplete drying of a sample (for example, the incomplete removal of a recrystallization solvent) have on the melting point? 7. Why should the melting point sample be finely powdered? 8. You suspect that an unknown is acetanilide (mp 113.5-114'C), Give a qual- itative 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 90-93C. Is your unknown compound A with a reported melting point of 95.5-96°C or com- pound B with a reported melting point of 90.5-91 C? Explain. 10. Why is it important to heat the melting point bath or block slowly and 11. Why is it important to pack the sample tightly in the melting point 12) An unknown compound is suspected to be acetanilide (mp 113.5-114c). 13. Which would be expected to have the higher boiling point, t-butyl alcohol 14. When borosilicate glass (Kimax, Pyrex), 20 1.474, is immersed in a solu- steadily when the temperature gets close to the mp? capillary? What would happen to the mp if this unknown were mixed with (a) an equal quantity of pure acetanilide? (b) an equal quantity of benzoic acid? (2-methyl-2-propanol) or n-butyl alcohol (1-butanol)? Explain. tion having the same refractive index, it is almost invisible. Soft glass with n20 1.52 is quite visible. This is an easy way to distinguish between the two types of glass, Calculate the mole percents of toluene and heptane that will have a refractive index of 1.474 assuming a linear relationship between the refractive indices of the two.

Explanation / Answer

2. Generally, when we increased the impurity, then the melting point would be lower than it was earlier.

3. Three tubes having same melting point can be distinguished by mixing unknown substance in tubes with known substances. After, that melting point of mixed sample is taken. If the melting point is lower than initial reading and wider than three of them are different compounds otherwise all the three are same.

5. Melting point in incorrect because solid does not melt at specific temperature but melt at range of temperature. Hence, it is more useful to say as melting point range.

6. Incomplete drying will lead to the increased of meting temperature

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