Who Has My Compound Set A

A prelimary investigation reveals that an unknown material has a melting range around 250 °C. The heating curves for a melting point apparatus are shown below. Use the curves to determine what voltage would be required to produce a temperature change of 2 °C/min close to the needed melting point. What voltage must the melting point apparatus be run at in order to produce a 2 °C/min change near 250°C? Enter the votage to the nearest 5 volts.

65 Volts Note that the second graph gives the relationship between voltage and temperature for a given rate. Using the graph on the right, find 250 °C on the vertical axis then read across until you reach the second curve, ascribed to 2 °C/min, then read vertically down to find the voltage as 65 V.

During a melting point determination, it is normal to first take a rapid rough measurement followed by a slower more accurate measurement. Why is it important to allow the Mel-Temp to cool 15 °C before placing the second capillary tube in, and to set the voltage generate 1-2 °C temperature increase per minute?

A rapid temperature increase will give a melting point range that is too high. When observing the second tube, it is important to allow the Mel-Temp to cool 15 °C before placing it in and set the voltage setting to 1-2 °C temperature increases per minute because a rapid temperature increase will give a melting point range that is too high.

Why is it good practice to perform a mixed melting point determination at two different ratios of unknown to known materials in the melting point capillary?

Because it is possible that one particular combination of materials generates a eutectic. A eutectic is a mixture that produces a low, sharp melting point. It is possible to mistake the sharpness of a eutectic as a pure sample, and thus misjudge the temperature reading.

Which of the following statements about melting point determination is not true? A. Pure substances typically have a melting range of less than 2 °C. B. The upper limit of the melting temperature range occurs when the last remaining crystal melts. C. The presence of impurities introduces entropy, which raises the energy required to melt the substance. D. The lower limit of the melting temperature range occurs when the first crystal starts to melt. E. The melting point is a physical measure of the forces holding the material into its crystal lattice.

C. The presence of impurities introduces entropy, which raises the energy required to melt the substance. The melting range for pure materials, measured from the first observed sign of melting to the last observed sign of remaining solid, is typically less than 2 °C. Impurities increase the entropy and reduce the energy required to cause melting. Literature values are often just the upper limit of the melting range.

When comparing melting temperature ranges of pure and impure samples, which of the following statements describes the melting temperature range of an impure sample? For a pure substance: For an impure substance:

For a pure substance, the melting temperature range is narrower than that of an impure sample. For an impure substance, the melting point is lower than that of a pure sample. The addition of impurities to a substance causes a depression and broadening of the melting point.

Mixture melting point determination, solubility tests, and thin layer chromatography can be used to assess the purity and identity of a substance. Drag and drop each of the chemical wastes generated in this reaction into the appropriate container.

Glass Waste: used melting point capillaries Solid Waste and Used TLC Plates: contaminated paper towels 10% Aqueous Sodium Bicarbonate Waste: sodium bicarbonate solution Hexanes, Ethyl Acetate, Ethanol, Acetone Waste: TLC solvent Melting point capillaries, must be disposed of in a glass waste container. Once a paper product is contaminated it can no longer be disposed of in the trash, and must be placed in the appropriate Solid Waste and Used TLC Plates container. The sodium bicarbonate solution is mostly water, and it should be disposed of in an 10% Aqueous Sodium Bicarbonate waste container. The TLC solvent is a mixture of ethyl acetate and hexanes; it should be disposed of in the Hexanes, Ethyl Acetate, Ethanol, and Acetone waste container.

List the seven procedural steps, from start to finish, that are required to perform an accurate melting point determination.

Press the open end of a capillary tube into the solid sample. Invert the tube and tap gently to move the solid to the bottom of the capillary. Place the tube with sample into the melting point apparatus, and heat at 10 degrees per minute. Observe the material through the complete melting process to get a rough estimate of the melting range. Turn off and allow to cool by 15 degrees. Replace the existing tube with a new sample. Heat at 2 degrees per minute through the expected range. Solid is loaded into the capillary by pressing its open end into the sample. The sample is moved to the bottom end of the tube by tapping it gently. The sample tube is placed into the melting point apparatus, and a quick practice melting range determination is made at a high rate of temperature change. The experiment is stopped at the first signs of melting, and the temperature is recorded. The melting point apparatus is turned off and allowed to cool by 15 °C. Using a new sample, the procedure is repeated at a rate of temperature change of 2 °C/min for a more precise determination of melting range.

Which of the following statements best describes eutectic composition?

The composition of a mixture of two components that generates the lowest possible melting point. A eutectic composition occurs when a single composition of a two component mixture melts at a temperature that is lower than either of the melting points of the two pure components.

How would the melting range of a pure organic material be changed if there was a tiny shard of broken glass inside the melting point capillary?

The melting range would not be affected. The addition of impurities to a pure substance will normally result in a depression and broadening of the observed melting range. Because the tiny shard of glass does not have any appreciable solubility in the liquid phase of the organic material, it will not change the observed melting range.

Adding a strong ionic electrolyte, like sodium chloride, to the aqueous solution removes traces of water from the organic layer and decreases the solubility of organic molecules in the aqueous layer.

True