When 2.0 moles of aluminum reacts completely with 3.0 moles of
chlorine gas (Cl2) to form aluminum chloride, the total number of
moles of the substances in the system:
a. increases
b. decreases
c. remains the same
d. cannot be calculated

Hi, teatea-ga:

If I understand the question, 2 moles of (monatomic) aluminum react
completely with 3 moles of (diatomic) chlorine gas to form 2 moles of
aluminum chloride (AlCl3).

[Aluminum chloride - formula and properties]
http://www.chemicalland21.com/arokorhi/industrialchem/inorganic/ALUMINUM%20CHLORIDE.htm

Therefore we go from having 2 moles of aluminum and 3 moles of
chlorine gas to having just 2 moles of aluminum chloride.  The total
number of moles "of the substances in the system" drops from 2+3=5 to
only 2 moles.

Then the answer is B. decreases.

regards, mathtalk-ga


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