why can't grams (g) be used to determine which substance is the limiting reactant?

Question

For instance if we have CaO+H2O->Ca(OH)2
and want to find out how much caclium hydroxide will be
produced when 10g of calcium oxide reacts with 3g of water?
What is the reason/s why we cannot do a direct gram comparison instead of converting to moles to find the limiting reactant? Please help thanks!

Answer 1

because substances normally have different molecular weight, so 1 gram of H20 does not have the same number of molecules than 1 gram of CaO, Suppose you have 10g of calcium oxide and 5 grams of water. If you see mass only, you may (erroneously) conclude that water is the limiting reactant. But the truth is that you will have;

n=m/Mw

10/56.1 = 0.1782 moles of CaO (that's about 1.07e+23 molecules)

and

5/18 = 0.2778 moles of H2O (Around 1.6778e+23 molecules)

The reaction stoichiometry establishes that, theoretically, for every molecule of CaO there must be one molecule of water to react completely and will yield one molecule of Ca(OH)2. But for the example above, there is not enough CaO to react with the water available, even though CaO weight is higher than H2O. So, CaO is the limiting reagent, and, if CaO reacts completely, you will produce 0.1782 moles of Ca(OH)2 or...

m=n x Mw = 0.1782 * 74.093 = 13.2 g of Ca(OH)2

and you will have 0.2778 - 0.1782 = 0.0996 moles of water unreacted (remaining) = 1.79 g.

Repeat these calculations for the example you give and see what happens. Hope you understood.

Answer 2

Because each compound has a different molar mass (grams per mole) A mole of CaO is not the same mass as a mole of Calcium hydroxide. The chemical equation only gives the molar ratios for the reaction. A gram to gram comparison would give 3g of calcium hydroxide since water would be limiting. But this is not correct. What you really have is 0.18 mol of CaO and .09 mol of water. Since it's a one to one ratio, you have only enough water to make 0.09 mol of calcium hydroxide. That would be about 6.66 grams of calcium hydroxide - not even close to 3g if you used grams only. You have to compare apples to apples - moles are not the same as grams. It would be like saying a dozen bowling balls has the same mass as a dozen eggs! Sure they are both a dozen but the grams don't match.

Answer 3

It reacts in moles. 1 mole of CaO reacts with 1 mole of H2O. The moles have different masses.