Chemistry Units and Ratios, Part 4 - Molar Mass

Greetings,

A chemist uses molar mass to determine the amount of a compound to weigh out for a chemical analysis and for making a solution of a certain molarity.  Molar mass is a derived unit formed by the ratio of a compound mass in grams to 1 mole of it.  If the number of moles contained in a certain gram-mass is known, molar mass can be calculated.  This would be most useful for a quality control analysis and to help confirm the identity of a newly synthesized experimental compound: A Mass Spectrometer (MS) is an instrument that can be used for this.  With MS, a sample is bombarded with a high-energy electron beam causing compound molecules to ionize/break apart.  The ionized molecule fragments then move through the instrument where they are separated according to their mass-to-charge ratios, most commonly by quadrupole magnetic fields or time-of-flight.  A detector provides a count of each individual mass fragment and sends the resulting information to a computer where a mass-spectrum is prepared.  A mass-spectrum is basically a graph of fragment number vs. fragment mass/charge ratio.  A properly interpreted mass-spectrum will provide the molar mass of a pure compound and a "fingerprint" for a particular molecular structure.  More commonly, a chemist will determine the molar mass of a given or known compound by combining the individual molar masses of elements making up the compound.  An element's molar mass is taken directly from the periodic table.  A prior post, Molar Mass Using the Periodic Table , provides a useful example.  This post concludes with a drawing showing a graphical representation of the molar mass unit.

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