Exp 3 Recitation
What is the area of ideal solubility?
0.1-0.3, 24.99 is too high (it will dissolve a lot), and 0.0009 is too little (won't dissolve)
Would a small amount of phenacetin (m.p. 135 °C) in a sample of predominantly acetanilide (114 °C) increase or decrease the melting point and why?
Decrease. Even though it has a higher melting point, impurities or different substances always lower and broaden the range
What does it mean when something has low solubility?
It may not dissolve
What does it mean when something has very high solubility?
It probably won't stay solid at low temperatures
Why is the melting point reported as a range? How is the range decided?
Impurities can cause things to melt at a range. So, the range is from the first drop of liquid to when it is all liquid
Why would we use capillary tubes filled with such a small amount of solid to determine the unknown's melting point as opposed to observing larger amounts of solid melt on a hot plate?
Large amount would just take longer and amounts don't influence melting point
Why should you obtain at least two melting point readings per sample? If your first two melting point values don't agree, what explanations might you suggest?
More accurate reading. If something is different, it can mean impurities
Why would you allow the filtered liquid to cool slowly? What would happen if you did not allow slow cooling?
Slow cooling allows for more pure crystals. If you cool fast, impurities could be in the crystals
Why are you asked to measure the melting point of the unknown, the unknown mixed 1:1 with pure acetanilide, and the unknown mixed 1:1 with pure phenacetin?
So you can see the actual value of the unknown. And then see when it is mixed with something, if it doesn't change, it is the same
How do you decide whether to use enough hot water to dissolve acetanilide or enough to dissolve phenacetin?
Use the one that takes a lesser amount. You don't want to overshoot it
Suggest 3 reasons why we employ water instead of ethanol as the recrystallization solvent for the unknowns?
Water is not flammable, it is cheaper, it also doesn't have a really high boiling point like ethanol
Why and when would you put the recrystallizing solution in an ice bath?
When its a room temp. Its safer so a violent reaction doesn't happen