Examples
Example 1
A 50.6 g sample of Mg(OH)2 is reacted with 45.0 g of HCl the balanced equation is shown as: Mg(OH)2 + 2 HCl --> MgCl2 + 2 H2O
Mg(OH)2 is r1, 2HCl is r2 (r=reactant)
GFW of r1=24(Mg)+32(O2) +2(H2)=58
GFW of r2=1(H)+35.5(Cl)=36.5
Moles of r1=50.6g/58(GFW)=0.8724 mol
Moles of r2=45g/36.5(GFW)=1.2324 mol
Total value r1=0.8724 mol X 2(coefficient of r2)=1.7448 mol
Total value r2=1.2324 mol X 1(coefficient of r1)=1.2324 mol
Since r2 is less than r1 it is the limiting reagent, so HCl is the limiting reagent.
Example 2
What would be the limiting reagent if 34.3 grams of NO2 were reacted with 5.78 grams of H2O? Balanced equation: 3NO2+H2O=>2HNO3+NO
NO2 is r1, H2O is r2 (r=reactant)
GFW of r1=14(N)+32(O2)=46
GFW of r2=12(H2)+16(O)=28
Moles of r1=34.3g/46(GFW)=0.745 mol
Moles of r2=5.78g/28(GFW)=0.206 mol
Total value r1=0.745 mol X 1(coefficient of r2)=0.745 mol
Total value r2=0.206 mol X 3(coefficient of r1)=0.618 mol
Since r2 is less than r1 it is the limiting reagent, so H2O is the limiting reagent.
A 50.6 g sample of Mg(OH)2 is reacted with 45.0 g of HCl the balanced equation is shown as: Mg(OH)2 + 2 HCl --> MgCl2 + 2 H2O
Mg(OH)2 is r1, 2HCl is r2 (r=reactant)
GFW of r1=24(Mg)+32(O2) +2(H2)=58
GFW of r2=1(H)+35.5(Cl)=36.5
Moles of r1=50.6g/58(GFW)=0.8724 mol
Moles of r2=45g/36.5(GFW)=1.2324 mol
Total value r1=0.8724 mol X 2(coefficient of r2)=1.7448 mol
Total value r2=1.2324 mol X 1(coefficient of r1)=1.2324 mol
Since r2 is less than r1 it is the limiting reagent, so HCl is the limiting reagent.
Example 2
What would be the limiting reagent if 34.3 grams of NO2 were reacted with 5.78 grams of H2O? Balanced equation: 3NO2+H2O=>2HNO3+NO
NO2 is r1, H2O is r2 (r=reactant)
GFW of r1=14(N)+32(O2)=46
GFW of r2=12(H2)+16(O)=28
Moles of r1=34.3g/46(GFW)=0.745 mol
Moles of r2=5.78g/28(GFW)=0.206 mol
Total value r1=0.745 mol X 1(coefficient of r2)=0.745 mol
Total value r2=0.206 mol X 3(coefficient of r1)=0.618 mol
Since r2 is less than r1 it is the limiting reagent, so H2O is the limiting reagent.