Stoichiometry (continued)

Excessive and limiting reagent calculation

To ensure that the reaction occurs and to occur faster, excess reagent is usually added. Only one of the reagents will be in excess. The other reagent will be the limiting.

These calculations can be identified when the problem has two reagent values. It is necessary to calculate which of these reagents is the limiting agent and which one is the excess.

Once the limiting and excess reagent is discovered, only the limiting agent is used as the basis for stoichiometric calculations. Examples:

1) Zinc and sulfur react to form zinc sulfide according to the following reaction:

Reacted 30g of zinc and 36g of sulfur. Who is the excess ruler?

Balance the chemical reaction:
Zn = 30g
S = 36g
Turn the mass in grams to mol:


By the proportion of reaction 1mol of Zn reacts with 1mol of S. So 0.46mol of Zn reacts with how many moles of S? A rule of three can be made to check which conductor is in excess:

x = 0.46mol of S

So 1mol Zn needs 1mol S to react. If we have 0.46mol of Zn, we need 0.46mol of S, but we have 1.12mol of S.

We conclude that S is in excess and therefore Zn is the limiting ruler.

2) How many grams of ZnS will be formed from the data in the above equation?

To solve this question, only the limiting reagent value is used.


x = 44.68g ZnS

Some useful constants and conversions

1atm = 760mmHg = 101325Pa
1Torr = 1mmHg

R = 0.082 atm.L / mol.K
R = 8.314 / mol.K
R = 1.987cal / mol.K

Avogadro Number: 6.02.1023

1ml = 1cm³
1dm³ = 1l = 1000ml

1000Kg = 1ton
1Kg = 1000g
1g = 1000mg

1nm = 1.10-9m