CHM205 Lab Final Study Guide

(Expt 0): Define "melting point range" based on the two values that are used to describe a compound's melting point.

the melting point range is the span of temperatures over which a substance melts. the lower temperature is wehre the substance begins to liquify & the upper temperature is where it is completely liquified

(Expt 0): What is the proper heating rate used to determine a compound's melting point range?

1-2*C per minute

Why is it important to keep your Pasteur pipet in a vertical position?

you may lose several drops or the entire sample in the pipet if you don't

What is the purpose of the filtration of a sample?

either to isolate a desired solid or liquid or remove an unwanted solid or liquid

Filtering pipet

this filter pipet is made by pushing a pea-sized amount of loose cotton into the constriction of the pipet using a metal wire. this type of pipet is most commonly used as a filter to remove an unwanted solid from a liquid

Filter-tip pipet

used to remove solids from very small amounts of liquid by leaving the solid behind (as opposed to trapping it). this pipet is prepared by rolling a very small piece of cotton into a tiny cylinder, then dropping it into the large end of. apipet and pushing the cotton into the tip of a pipet with a metal wire.

Melting point

a common method used to establish the qualitative purity and possible identity of an organic compound. The melting point of a compound is an intensive property, so it can be used to identify the compound.

Mixed melting points

- technique to identify your unknown compound

- idea behind this procedure is that if two compounds (your unknown & a known compound) are "mixed" together and a melting point is taken, a change in the melting point temperature and range for the solid mixture will not be observed if the unknown & known are identical compounds

- if your unknown and a known compound are not identical, the melting point temperature of the solid mixture will be lower and the melting point range broader

mixed melting point depressioin

when you mix two different compounds, their melting point drops and broadens compared to the pure sample

if you mix a compound with itself, the melting point stays sharp & unchanged

(Expt 1): The solubility of an unknown compound X in ethanol is found to be 0.21 grams per 10 mL at 0C and 1.4 grams per 10 mL at 78C. What is the minimum amount of ethanol needed to dissolve a 2.0 gram sample of compound X at 78*C? After recrystallization of the 2.0 gram sample in an ice bath, how much of compound X will remaind dissolved in the cooled solvent (not recovered)?

minimum ethanol needed:

- known: solubility of compound X in ethanol = 1.4g per 10mL at 78*C

- ratio of solublity to volume: 1.4g/10mL = .14g/mL

- volume of ethanol needed = mass/solubility = 2g/(.14g/mL) = 14.29 mL

- at 0*C, ratio of solubility to volume = .21g/mL x 14.29 mL = .3g

Recrystallization

the process of purification that involved dissolution of a solid in a hot solvent, filtration of the heated solution or mixture, crystal formation,a nd the isolation of the crystalline compound

Factors that dictate the solubility of a solid

1. "Like dissolves Like."

2. Intermolecular Forces

3. Temperature

4. Molecular Structure

Factors that dictate the crystal formation of a solid

1. Degree of Supersaturation (as temp drops, solubility decreases --> molecules come out as crystals)

2. Slow Cooling

3. Proper solvent choice

4. Intermolecular interactions

What does crystal formation depend on?

controlled supersaturation and proper molecular alignment

True or false: Always add a couple of boiling chips before heating luquid to their boiling point.

True

When to add charcoal to a boiling solution?

Never add the charcoal to a boiling solution, as this is likely to cause the solution to boil over