Question #69b1e

Start by looking at the chemical equation given to you

#"Ca"_ ((s)) + color(blue)(2)"HCl"_ ((aq)) -> "CaCl"_ (2(aq)) + "H"_ (2(g)) uarr#

The balanced chemical equation tells you the ratio that must exist between the number of moles of each chemical species that takes part in the reaction.

In this case, you can see that it takes #color(blue)(2)# moles of hydrochloric acid to react with #1# mole of calcium metal. At the same time, the reaction produces #1# mole of calcium chloride and #1# mole of hydrogen gas.

Simply put, the number of moles of hydrochloric acid that take part in the reaction will always be twice as high as the number of moles of calcium chloride produced by the reaction.

In your case, #4# moles of hydrochloric acid are said to react. This means that the reaction will produce

#4 color(red)(cancel(color(black)("moles HCl"))) * overbrace("1 mole CaCl"_2/(color(blue)(2)color(red)(cancel(color(black)("moles HCl")))))^(color(purple)("given by the balanced chemical eqaution")) = color(darkgreen)(ul(color(black)("2 moles CaCl"_2)))#