And finally, what do we replace the question marks with? Well
that's the mole ratio from the balanced equation. There is a
coefficient of 1 in front of the Mg(OH)2, and a 2 in front of the
HCI in the equation, so we have:
4 mol HCl x
1 mol Mg(OH)2
2 mol HCI
=
2 mol Mg(OH)2
As you can see, the units "mol HCI" cancel, and we are left
with the units "mol Mg(OH)2, which is what we want. The
multiplication is done by multiplying the number 4 by 1, and
then dividing by the 2. So to summarize, 4 moles of HCI will
require 2 moles of Mg(OH)2 in order to react completely.
Now try this one, using the same balanced equation: 2.6 moles
of Mg(OH)2 react with HCl, and there is plenty of HCI
available (we're not worried about that amount). How much
water (H₂O) will be produced as a product?