Kaden Fisher CHEM 341-12 PRELAB QUESTIONS EXPERIMENT 1 AND 2 (2.5 points each) 1. How will the melting point change if the solid sample is impure versus being pure? In other words, what differences would you expect to see in the melting point for an impure and a pure compound? If the sample is impure than one is more likely to see not only a lower melting point than that of the pure substance but also a wider range of a melting point due to the compound containing multiple substances that skew the range. An indicator of an impure compound is when the melting point range is greater than 2°C. 2. What does it mean if a mixed solvent system such as ethanol/water is suggested for a recrystallization? In other words, describe how you would recrystallize your solid. When one gets to the point of using a mixed solvent system it should mean that it was already tested for other solvents that the product could dissolve in but there was an issue initially where the solvent would not dissolve when hot, would dissolve when cold, or something else. This means that there is no practical solvent to use and one must use a mixed-solvent system such as ethanol and water seeing that these two are miscible. The benefits to this is that the at a high temperature the mixture will act as an alcohol and at low temperature it will behave like water and it is best in this case to use a product that is soluble in alcohol but insoluble in water 3. When checking the melting point of your impure sample, what temperature should you set as the starting temperature (the SET temperature) and what should the ramp rate be? For example, if the sample is believed to melt at 70 oC, at what temperature should you begin to heat the sample? The ramp rate is how fast the sample is heated per minute. Remember, for your impure sample, you simply want to identify possible compounds based on a rough estimate of the melting point. Well if you are lucky you can take one of the melting points from the unknown and do trial and error to see if that works but that honestly is not the most efficient thing to do. A good place to start most of the time is somewhere around 40°C never for certain but it is a good starting point. The goal is to get 20 C within your approximate melting point with a ramp rate of about 5-10 C per minute to have a rough estimate of where your sample's melting point falls. Once you record the temperature from this run you would start over with a new sample and get close to the pre-recorded temperature but this time with a ramp rate of 2 C per minute to have a more precise recording of the temperature. If the melting point was believed to be 70 C I would do an initial run starting around 50-60 C and then do a ramp rate of 5-10 C per minute until a more accurate measure is taken. If the sample does melt around that range I would start around 60 C but this time with a new sample and ramp rate of 2 C per minute. Kaden Fisher CHEM 341-12 4. Based on the melting point of your impure solid, you look at the given samples and believe you have compound A. Compound A is recrystallized using a mixed solvent system of ethanol and water. You add 1 ml of hot ethanol to your sample and it doesn’t completely dissolve. You continue to add hot ethanol but there isn’t any change. Some solid remains. What should you do next? If adding hot ethanol to it is not working then consider changing the composition by adding some water to it since it won’t be soluble in the water. The mixture should more readily dissolve in the ethanol now that you have shifted the composition slightly. Proceed to let it cool and pure product should crystalize while the impurities remain dissolved in it.
