Solving Stoichiometry
Problem
STEP-BY-STEP MASS TO MOLE
What is Stoichiometry
Stoichiometry is the study of the relationships or ratios
between two or more substances undergoing a physical or
chemical change
We will be taking steps to solve the
different equations
The type of problem we will look at is solving
for a Gram to Moles, Stoichiometry problem.
Lets start with the problem.
KBr+ Cl2
KCl + Br2
Potassium bromide reacts with chlorine gas to
produce potassium chloride and iodine gas. How
many moles of bromine gas can be produced
from 100g of potassium bromide
Solving Stoichiometry Problems
Step 1. Balancing the Equation
A balanced equation has equal numbers of
each type of atom on each side of the
equation.
Chemical equations usually do not come
already balanced. Making sure they are
balanced must be done before the equation
can be used in any chemically meaningful way.
So we need to look at our problem and make
sure that our equation is balanced
KBr+ Cl2
KCl + Br2
This equation is not balanced, so we need to
balance it.
2KBr+ Cl2
2KCl + Br2
Step 2: Find the Given
Potassium bromide reacts with chlorine gas to
produce potassium chloride and iodine gas.
How many moles of bromine gas can be
produced from 100g of potassium bromide
After reading the equation again, find what is
Given
We are given
◦ 100 g of Potassium bromide
We need
◦ Moles of bromine gas
Step 3: Look for the ratio of moles
Look for the Ratio of moles in the equation
If we have: 2KBr+ Cl2
2KCl + Br2
After the equation is balanced, the coefficient
basically is saying that that is how many moles
of that substance is needed to make the
reaction
Basically what is being said is that you need:
2KBr + 1 Cl2 to make 2KCl + 1Br2
These coefficients make up the mole ratio
◦ 2:1:2:1
So it takes 2 mole of KBr and 1 moles of Cl2 to
make 2 moles of KCl and 1 mole of Br2
Step 4: Lets Set up the Equation
The problem states If you start with 100g of Potassium Bromide,
how many moles of bromine gas will be produced?
So lets set up the problem :
Set up the T-Chart
We are given 100g KBr we are looking for
moles of Br2.
100 gm KBr
In order to solve this we need to find the
Molar Mass of KBr to get rid of grams of KBr (I
will round to the nearest whole number)
K-39
Br-80
Add them
up you get
119g of KBr
119g KBr
Step 4: Lets Set up the Equation
The problem states If you start with 100g of Potassium Bromide,
how many moles of bromine will be produced?
We know that 1mole of any molecule is equal
to the molar mass of that molecule:
Set up the T-Chart
So on top of the 119gm of KBr, I will have
100 g KBr
1mole of KBr
But I need to solve for Br2, so I need to cancel
my mole of KBr
1 mol KBr
119g KBr
Step 5: Look at my Mole Ratio
The problem states If you start with 100g of Potassium Bromide,
how many moles of Bromine gas will be produced?
As stated I need to solve for Br2, so I need to
cancel my mole of KBr
Set up the T-Chart
So now I will look at my mole ratio
◦ 2KBr+ Cl2
◦ 2: 1: 2: 1
2KCl + Br2
100 g KBr
I have 2 mole of KBr based off my ratio
I place that on the bottom so that I can cancel
mol of KBr
Then I look at what I need, I need moles of Br2
So I put the mole ratio for B2 on top
1 mol KBr
1 mol of Br2
119g KBr
2mol KBr
Step 6: Cancel Units and Do the Math
Set up the T-Chart
100 g KBr
1 mol KBr
1 mol of Br2
119g KBr
2mol KBr
0.42 mol Br2