Avogadro's Number, Moles, and Molecules One of the earliest academic challenges facing students in the first semester of general chemistry is to grasp the relationships existingamong atoms, molecules, grams, and moles. The huge value of Avogadro's number seems to numb the intellect and lead to some weird answers to typical problems, such as 1.0 X atom~lmole or 6.02 X 1 0 2 ~ / m o l . In order t o put these terms in proper perspective, this diagram can be very helpful. Each corner of the triangle is equivalent to any other corner but in different units; each side forms the basis for a simple equation relating the two ends and yielding three interrelated equations
X Atomic Weight (grams)
=-= Y
1.00 (moles)
&
I
A
/
z 6.02 x loz3 (atoms)
..~m ..
\
/ A
For example, let us take X = 1.00 g of sodium metal whose atomic E-w, t w ~ t s ~ ~ rac-ms~ weight is 23.0 and solve for Y which is found to he 0.0435 males of sodium. Solvine for Z in like manner yields 2.62 X l O Z 2atoms of the metal. As lone- as one of the three letters is known, the other two can be solved for. The three terms can be placed in any order around the triangle. In dealing with compounds, simply replace atomic weight by molecular weight and atoms by molecules. bvy
hy On
Thomas McCullough, CSC St. Edward's University Austin. TX 78704
Volume 67
Number 9
September 1990
783
