STOICHIOMETRY
Mass relationship of reactants and products
in a chemical reaction
 Hydrogen gas + nitrogen gas ----> ammonia gas
Qualitative: describes the “nature” of the
reactants and products
Quantitative: describes the amounts of
reactants and products
Steps for solving
Stoichiometry problems:
Write a balanced equation for the question.
Understand the mole and molar ratio
Convert mass of reactant to moles of
reactant.
Convert moles of reactant to moles of
product.
Convert moles of product to grams of
product.
Pick up the calculator and do the math!!
3 H2 + 1 N2
2NH3
What do the coefficients mean?
3 molecules : 1 molecule : 2 molecules
Does not equal:
3 grams : 1 gram : 2 grams
1N2 + 3H2
2NH3
1 molecules : 3 molecules : 2 molecules
10 molecules : 30 molecules : 20 molecules
100 molecules : 300 molecules : 200 molecules
1 (6x1023) : 3 (6x1023) : 2 (6x1023)
1 MOLE : 3 MOLE : 2 MOLE
1N2 + 3H2
1mole of
N2
:
molecules
3mole of
2NH3
:
H2 molecules
2mole of
NH3 molecules
This is the MOLAR RATIO
allowing you to solve problems
dealing with masses of
substances!
1 N 2 + 3 H2
2 NH3
If 75g of hydrogen reacts,
A) How many g of N2 reacts?
Use the molar ratio!!
First convert the grams to moles……
75g H2 x 1moleH2 / 2.02g H2
Next use molar ratio…..
75g H2 x 1mole x 1mole N2
2.02g H2
3mole H2
Now you’ve converted to moles of
N2, make the final conversion into
grams!!!
75g H2 x 1mole x
2.02g H2
1mole N2 x 28g N2
3mole H2 1mole N2
75g H2 x 1mole H2 x 1mole N2 x 28g N2
2.02g H2
3mole H2 1mole N2
= 346.5g = 350g of N2
1 N2 + 3 H 2
2 NH3
If 75g of hydrogen reacts,
B) How many g of NH3 are formed?
75g H2 x 1mole H2 x 2mole NH 3 x
2.02g H2
3mole H2
17g NH3
1mole NH3
MOLAR RATIO:
3mole H2 : 1mole N2 : 2mole NH 3
= 420g NH3
How about something a
little easier to follow????
Like………….
FOOD!!!!!
The Stoichiometry of a
sandwich…..
The Miller famous sandwich
consists of 2 slices of bread, 2
slices of deli meat, 1 slice of
cheese, and 3 slices of pickle.
Write an equation for the recipe…...
2 slices bread + 2 slices meat + 1 slice cheese + 3 slices pickle
1 sandwich
OR…..
2B + 2M + 1C + 3P
1B2M2CP3
How many Miller famous sandwiches
can be produced when 8 slices of
bread, 8 slices of meat, 4 slices of
cheese, and 12 slices of pickles are
available?
Break it down…..
BREAD: 8B x (B2M2CP3 / 2B)= 4 B2M2CP3
MEAT: 8M x (B2M2CP3 / 2M) = 4 B2M2CP3
CHEESE: 4C x (B2M2CP3 /1C)= 4 B2M2CP3
PICKLES: 12P x (B2M2CP3 / 3P)= 4 B2M2CP3
EVALUATION: WE CAN MAKE 4 COMPLETE
SANDWICHES!!
How many of each ingredient
is needed to make 10 Miller
famous sandwiches?
BREAK IT DOWN…..
BREAD: 10B2M2CP3 x (2B / B2M2CP3 )= 20B
MEAT: 10B2M2CP3 x (2M / B2M2CP3 )= 20M
CHEESE: 10B2M2CP3 x (1C / B2M2CP3 )= 10C
PICKLES: 10B2M2CP3 x (3P / B2M2CP3 )= 30P
THUS: 10 Sandwiches require 20B, 20M, 10C,
and 30P
Coming Up Next…….
Mass - Volume Stoichiometry
Instead of converting the moles of
unknown to grams you convert to
volume (L, ml, ul)
Volume - Volume - Stoiciometry
Instead of using the molar ratio you
use volume ratio and solve for volume