[PeaceLoveGlitter]

Hi everyone!

I'm working through two questions about limiting reagents, and I got the first, but the second is really confusing as it deals with 3 reactants, where my teacher only showed us how to solve with 2. Could anyone shed some light on this process? I have included the first question too so you guys can see my process!

Use mole ratios to identify the limiting reagent in the following reactions:
a) 2.75 mol of sodium mixes with 4.25 mol of water to form sodium hydroxide and hydrogen gas.

Balanced equation:
2Na + 2H2O -> 2NaOH + H2

Number of moles of each reactant:
Na=2.75 mol      (H2O)=4.25 mol

Mole ratio between reactants (assume that all of the H2O did react):
In the reaction, 2 mols of H2O requires 2 mols of Na to react
   4.25 mols H2O will require z mols Na to react
   z=4.25 mol × (2/2) =4.25 mol 

The water needs 4.25 moles of sodium, yet there is only 2.75 moles available. There will not be enough sodium to use all the water; sodium is the limiting reagent.

b) 2.5 mol of CaO mixes with 3.0 mol of SO2 and 2.0 mol of O2 to produce CaSO4.

Balanced equation:
2CaO + 2SO2 + O2  -> 2CaSO4

Number of moles of each reactant:
CaO=2.5 mol      SO2 =2.75 mol      O2 =2.0 mol

I know the next step is the actual calculation part, and I have determined that the limiting reagent is CaO, but I dont know how to show it!

Thanks in advance!

[Babcock_Hall]

There is no fundamental difference whether there are two reagents or three.  However, just calculating the number of moles of each reactant is not sufficient and is often misleading; you must use the stoichiometric coefficients to calculate the number of moles of product that each reactant would make.  In example (a), the stoichiometric coefficients of Na and of water were equal, so one could ignore this issue.  You may have done this already, in that you have identified CaO as the limiting reagent.

[Enthalpy]

As you already believe that CaO is the limiting reactant, tell how many times the available CaO lets apply the balanced equation, and check that the amounts of SO₂ and H₂O suffice for that.