Question

Powder (shor sten FUNNELS Fitering (long stem 63 Chapter 4 Melting Points, Boiling Points, and Refractive Indices Questions 1. What effect would poor circulation of the melting point bath liquid have on the observed melting point? 2. 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? 5. Strictly speaking, why is it incorrect 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. 9, You have an unknown with an observed melting point of 90-93°C. 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 steadily when the temperature gets close to the mp? 11. Why is it important to pack the sample tightly in the melting point capillary? 12. An unknown compound is suspected to be acetanilide (mp 113.5-114°C). 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? 13. Which would be expected to have the higher boiling point, t-butyl alcohol (2-methyl-2-propanol) or n-butyl alcohol (1-butanol)? Explain. 14. When borosilicate glass (Kimax, Pyrex), n0 1,474, is immersed in a solu- tion having the same refractive index, it is almost invisible. Soft glass with n 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.

Answer 1

1.) It will going to give wrong experiment result :

This is because the measured temperature is actually at the position of the thermometer, not at the sample, so the sample's temperature may be different from what is indicated.

So if the circulation is low high melting point is observed and if fast then low Melting point is observed .

2) Since it is insoluble it will don't have any significant effect on Range of Melting Point .It will remain as it is .

3). This can be done by Mixed Melting Point Analysis

The phenomenon of melting point depression can be applied to the identification of unknown pure substances. For Example

If you measure the melting point of a sample at 60 °C, Finding it from MP tables that this is the same melting point for several different reference compound. The substance can be identified by determining its Mixed Melting Point

Thesample is mixed one-by-one with small amounts of the references and the mixed melting point is determined in each case. Whenever the melting point of the sample is depressed by mixing a small amount of a reference with it, the two substances cannot be identical.

If, the melting point of the mixture does not drop, the reference substance that was added was identical to the sample .

In this way this procedure can be applied to Three compounds and can be easily identified.

4). If the heating is done to Rapid then the Wider range of Melting point wil be observed as compared to True Boiling Point of Your compound .

Because Sample takes some time to absorb and Mercury will rise so Fastly so Altering difference would be there .