Mathematics and Formulas
 The mass of an atom if a relative value based on
the mass of a Carbon-12 atom
Vocabulary
 Formula Mass: the smallest unit of the compound,
which is the sum of the atomic masses of all the
atoms present.
 Gram Formula Mass: is the formula mass
expressed in grams instead of atomic mass units.
 Gram Molecular Masses: expressing the gram
formula masses of molecular substances.
How to Calculate GFM
 H2O
GFM= (1 x 2) + 16 = 18 grams (g)
 Al2(SO4)3
GFM= (2 x 27) + (32 x 3) + (12 x 16)= 342 grams (g)
Moles and Avogadro’s #
 Mole: a unit of measure. It is the number of atoms of
carbon present in 12.000g of Carbon-12
 Molar Mass: the mass in grams of one mole of a
substance
Converting: Grams to Moles
 How to formula:
Moles = given mass
gram formula mass
(found on the periodic table)
Example: 36 g of water
Moles: 36/18= 2 moles
Find Moles!
 Question: How many moles are equivalent to 4.75
grams of sodium hydroxide (NaOH)
Given: Mass of NaOH = 4.75 grams
~First, calculate the formula mass of sodium hydroxide
Na: 2 atoms x 23.0 amu/atom= 23.0 amu
O: 1 atom x 16 amu/atom= 16 amu
H: 1 atom x 1 amu/atom= 1 amu
TOTAL: 40.0 amu
Find Moles: Continued
~Next, calculate gram formula mass
gram formula mass = formula mass in grams
So… gram formula mass of NaOH = 40.0 grams
~Last, use the gram formula mass to convert the given mass
to moles:
Moles= number of grams
Gram formula mass
Moles NaOH= 4.74 grams
40.0 grams
ANSWER: 0.119 mol
Converting: Moles to Grams
 How to formula:
Grams= number of moles X gram formula mass
Find Grams!
 Question: How many grams are present in 40.5 mol
of sulfuric acid (H2SO4)?
Given: Moles of H2So4= 40.5
~First, find formula mass of H2SO4
H: 2 atoms X 1 amu/atom = 2 amu
S: 1 atom X 32.1 amu/atom = 32.1 amu
O: 4 atoms X 16 amu/atom = 64 amu
TOTAL: 98.1 amu (amu=grams)
Find Grams: Continued
~Next, get the grams (hint: its exactly the same as amu,
just put grams!!)
~Last, convert:
Grams of sulfuric acid= 40.5 moles X 98.1 =
Answer: 3970 grams
Using Avogadro’s Number
 Avogadro’s Number is used to find the number of
molecules in one mole.
REMEMBER!
 If the question asks for “atoms” or “moles”,
you will be using Avogadro’s Number some
how! To find molecules…times moles by
Avogadro's
number!
Your Turn!
2 moles of Hydrogen gas
2 x (6.02x
23
10 )
= 1.2 x
24
10
Water in compounds
 Hydrates: crystals that contain attached water
molecules.
 Anhydrous: substances without water molecules
present.
Percentage Composition
 Formulas represent the composition of a
substance, and the percentage composition
represents the composition as a % of each
element compared to the total mass of the
compound.
Percent is just a piece of
something!
 So when trying to find the percent
composition, you are just trying to find out
how much one element contributes to the
overall compound.
Percent Composition Formula!
Piece
Whole
100= %
Remember this!!! It is important
Percent Composition
 Formula: KClO3
 Question: What percent is Oxygen?
 How to solve:
First write out the amount of each atom and multiply it by
the mass on your periodic table:
K= 1 atom X 39.1 amu= 39.1 amu
Cl= 1 atom X 35.5 amu = 35.5 amu
O= 3 atoms X 16.o amu = 48.0 amu
Next add up all the amu values to get the total mass of the
compound.
39.1 + 35.5 +48 =122.6
Last put the element you are looking for over the total
compound value and multiply by 100 to get %
(48/122.6) x 100= 39.2 %
Finding Molecular Formulas from
Empirical Formulas
 We know that empirical formulas tell us the simplified
version of any molecular formula. So, to find the molecular
formula from the empirical formula you must find the
common denominator to multiply by.
 Easy way to remember formula starting from empirical
formulas:
Molecular Formula =Molecular Mass ( Amu/grams given)
x subscript #s
Empirical Mass (Amu/Grams periodic table)
Continued…
 Example: if you have CH2 (most simplified) which has
a molecular mass of 14 amu, but you are told that your
molecular formula should have a compound on 42
amu. Simply, place the number you wish to have over
that you currently have from your empirical
compound: 42/14= 3. Take that integer of 3 and
multiply all of the sub numbers in your compound by
it….giving you: C3H6
WARNING!!!
If you do not get a whole number when finding the
integer to multiply by, then you did something wrong!!
Most likely you just switched your masses of empirical
and molecular!
Find Molecular Formula!
Question: What is the molecular formula for a
compound with a molecular mass of 180 amu and an
empirical formula of CH2O?
~ First, find the molecular mass of CH2O:
C: 1 atom x 12 amu/atom= 12 amu
H: 2 atoms x 1 amu/atom= 2 amu
O: 1 atom x 16 amu/atom= 16 amu
TOTAL: 30.0 amu
Continued…
~Next, divide the molecular mass by the empirical mass:
180 amu = 6
30 amu
~Last, multiply the subscripts in the empirical formula
by 6 to get the molecular formula:
C1x6H2x6O1x6 =C6H12O6
** This is the chemical formula for Glucose sugar!!
Interpretation: Moles
Mole to Mole Calculations
Example:
How many moles?
Question: How many moles of water (H2O) will be
produced from 3 moles of ethane in the equation
below?
2C2H6 + 7O2
4CO2 + 6H2O
~First, use the balanced equation to determine the mole
to mole ratio of ethane: water = 2:6
~Next, Set up a proportion between the known and
unknown values:
In question
3 mol C2H6 =
x
In given Equation
2 mol C2H6 =
6 mol H2O
Remember!!!
 When solving problems, ALWAYS write what you
know (what you are given) and what you need to find.
This helps you to know what formula to use to solve
the problem.
Moles……
Last, multiply 3 x 6 and divide that number by 2!
(3x6)/ 2 = x
18/2=x
x=9 moles of H2O
Mole Relations in balanced
equations
 We know that equation’s coefficients can give us a
great amount of information about the elements and
compounds listed.
 Balanced equations show ratio of moles in formula:
Ex:
Equation: 2C2H6 +7O2 = 4CO2 + 6H2O
Moles C2H6
Moles O2
Moles CO2 Moles H2O
In equation 2
7
4
6
4
14
8
12
If x’s 2
3.5
2
3
If dividied 1
by 2
YOU FINISHED!!!!
Not a mole,
but still
really cute!!
Resource :
http://library.thinkquest.org/10429/low/indexl.htm