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Chemistry 303: Tips for the assignment

Assignment Tips

Take note of some common sticking points in completing today's lab assignment:

  • Not Applicable: If density or boiling point data isn't available you can write "N/A".
  • Hazards: Check the SDS (sect. 22 & 21, hazards statements are helpful). Summarize & write out in words.
  • 2A & 2B Solubility: Write out in words, such as "Insoluble in ethanol" -- do not use abbreviations.

Liquid or Solid?

CRC sometimes and Aldrich rarely tells you directly whether a compound is liquid or solid at room temperature and pressure. The most reliable way to determine this is by comparing the melting and boiling points to room temperature (RT). RT is 20-25 C, depending on the source and the day.  Think of it as though you had a chunk of the compound.  If you leave that chunk out on the lab bench at room temperature, what will happen to it?

If the melting point is below RT, the chunk of compound will have melted below RT, so it is a liquid. Actually, it could also have boiled below RT and been a gas, so check the boiling point, as well. If the boiling point is above RT, the chunk will still be liquid at RT.  If the melting point is above RT, the chunk of compound will not have melted below RT, so it is a solid.

First:

  • Melting point above 20-25 C --> solid
  • Melting point below 20-25 C --> liquid or gas

Then:

  • Melting point below 20-25 C AND boiling point above 20-25 C-->liquid
  • Melting point below 20-25 C AND boiling point also below 20-25 C--> gas
Heating-Curve

CAS Numbers

CAS numbers, also called Registry Numbers (RNs), are important for finding information about chemicals. These numbers are assigned by Chemical Abstracts Service (CAS) to identify a unique “arrangement of atoms.” Most chemical literature has a CAS number index, so once you have a CAS number from one source, you can use it to find the same compound in another source. Even Google uses CAS numbers!

Two chemicals with the same CAS number are the same thing; so iso-butane and 2-methylpropane both have the CAS number 75-28-5 and are two names for the same compound. 2-Butanol has three CAS numbers: 72-92-2, 4221-99-2, and 14898-79-4; there are two isomeric forms of 2-butanol, and the numbers represent the racemic mixture, the (S)-(+), and the (R)-(-) form, respectively. Similarly, isotopically labeled or ionic forms may also have their own CAS numbers.

CAS numbers always have the format “some number of digits - dash - two digits - dash - one digit”; the format doesn’t mean anything except that it doesn’t look like any other kind of standard number. CAS numbers are assigned sequentially, like Social Security numbers, so you can’t tell anything about the structure from the CAS number, except whether two compounds are the same or different (above), or how long the compound has been known. Compounds recorded earlier in the chemical literature will have shorter CAS numbers than compounds more recently registered (like the 2-butanol example, above).