Are fewer steps required in solving stoichiometry problems?
In solving stoichiometry problems, there are quite fewer steps required but there are basic things we need to understand about stoichiometry.
Though stoichiometry could be a little confusing for some students which I feel it shouldn’t anyways but I want to show you the misnomer and an easy approach to stoichiometry calculations.
What are the steps required or taken in solving stoichiometry calculations?
The truth is that stoichiometry calculations are of different categories
Mole to mole stoichiometry
Mole to mass stoichiometry
Mole to volume stoichiometry
Mass to mass stoichiometry
Mass to volume stoichiometry
Volume to volume stoichiometry
So you need to master the art of solving these stoichiometry calculations.
5 fewer steps required in solving Stoichiometry
Discover the type of stoichiometry problems
The first step I recommend is for the candidate to discover what the examiner is asking.
Let me illustrate with few questions
N2 + 3H2====2NH3
Case 1
What amount of ammonia in moles would be evolved by 4 moles of Nitrogen?
This is mole to mole stoichiometry
Case 2
What mass of ammonia would be evolved by 4 moles of Nitrogen ?
This is mole to mass Stoichiometry
Case 3
What mass of ammonia would be evolved by reacting 4 grams of Nitrogen
This is mass to mass stoichiometry
Case 4
What volume of ammonia would be evolved by reacting 13 grams of Nitrogen.
This is mass to volume stoichiometry
Case 5
What volume of Nitrogen would be evolved by reacting 12cm3 of Nitrogen.
This is volume to volume stoichiometry
So , the first step is to deduce the type of stoichiometry problems you’re presented with.
Check and balance the chemical equation
Every stoichiometry calculation stems from a balanced chemical equation.
Of Course, there is a need to learn how to balance chemical equations if you’ve not mastered the art because it is the root of stoichiometry calculations.
Introduce mole to mole ratio comparison
When you’re certain the equation is balanced, you can thus introduce mole to mole ratio comparisons of reactants and products.
From the mole comparisons of balanced equations , you’ve already started dismantling the stoichiometry problems.
Please ensure you pick the coefficients of the substances because that’s the right mole to use in your calculations.
Example
If you are presented with this equation
2K2 + 2H2O === 2KOH + H2
Compare potassium to potassium hydroxide
2 moles of potassium ==2 moles of potassium hydroxide
Compare potassium to hydrogen
2 moles of potassium == 1 mole of hydrogen
Compare water to hydrogen
2 moles of water = 1 mole of Hydrogen
Start with the know parameter given to the unknown parameter you want to solve
At this moment, you will discover that in every stoichiometry practice questions and calculations, you need to first deduce the two substances required. All stoichiometry problems involve a given substance (reactant or product) and the unknown substance which is the one required to solve.
so from your mole ratio comparison which is the background information, you then go further to calculating the unknown or required substance.
Example 1
calculate the mass of potassium hydroxide produced when 3.3 moles of Potassium dissolves in cold water.
2K + 2H20 ====2KOH + H2
So you can see from the question that we have already gotten the amount given which is 3.3 moles of potassium , so we are looking for the mass of Potassium hydroxide that will be produced.
So to make the calculation easier, you can underline the two substances you’re comparing .
Going forward, I will compare the two substances
background information from balanced Chemical equations
2 moles of K ====2 moles of KOH
3.3 moles of K==== x
x = 3.3 x 2 = 6.6 moles of KOH
converting moles to mass
1 mole of KOH=56 grams of KOH
3.3 moles of KOH= x
x = 3.3 x 56 = 184.3 grams of KOH
Example 2
what mass of calcium chloride is produced by reacting 2.6 moles of Calcium according to this equation
Ca + MgCl2 =====CaCl2 + Mg
solution
Ca + MgCl2 =====CaCl2 + Mg
background information
2 moles of Ca====1 mole of CaCl2
2.6 moles of Ca=== x
x = 2.6/2 = 1.3 moles of CaCl2
converting moles to mass
1 mole of CaCl2= 111 grams of CaCl2
1.3 moles of CaCl2= x
x = 1.3 x 111 =144.3 grams of Calcium Chloride
Determine limiting reagent.
The last thing I will recommend is try and understand how to determine limiting reagents. Limiting reagent or reactant is the one that finishes first.
There are some stoichiometry problems in which the amount of two reactants will be given but one will be in excess while the other one will be the limiting reactant
I have fully explained how to determine limiting reagents before but I will just give a quick review.
Example
Calculate the mole of zinc sulphate produced when 26 grams moles of zinc react with 250cm3 of 0.05 mol/dm3 Sulphuric acid.
Zn + H2SO4 ====ZnSO4 + H20
Solution
compare the mole ratio of the limiting reagent and product required
You will see that there is a need to determine the limiting reagent since we are given the amount of two reactants.
Zn + H2SO4 ====ZnSO4 + H20
for zinc
n =m/M
n= 26/65 = 0.4 mole
for H2SO4
n =cv
n= 0.05 x 250/1000
n=0.0125 mole
Then divide by the coefficients
for zinc
0.4 /1 =0.4
for H2SO4
0.0125/1 =0.0125
The limiting reagent is the reactant with the least number of moles.
In conclusion, the fewer steps mentioned are helpful in solving stoichiometry problems of any degree and stoichiometry practice questions.