Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107	Fundamentals of Chemistry	Fall 2008
Lecture Notes: Lewis Dot Structures	© R. Paselk 2005
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How many moles of Cl^- are there in 2.36g CaCl₂?

First we need to determine the Formula Weight, which is simply the sum of the atomic weights of all of the atoms in the compound:

FW = AW_Ca + 2 (AW_Cl)

= 40.08 g/mol Ca + 2 (35.45 g/mol Cl) = 110.98 g/mol CaCl₂

Next we need to determine the number of moles of calcium chloride:

Moles CaCl₂ = (g CaCl₂) / (FW CaCl₂)

= 2.36 g / (110.98 g/mol) = 2.1265 x 10^-2 mol CaCl₂

Now we must determine the moles of chloride. From the formula we see that there are two atoms of Cl / CaCl₂. So:

Moles Cl = 2 (mol CaCl₂)

=2 (2.1265 x 10_-2) = 4.2530 x 10^-2.

But there were only 3 sig figs for the original mass, therefore

Moles Cl = 4.25 x 10^-2

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Last modified 21 September 2005