Stoichiometry Chapter 3 Notes for You
I. Isotopes
1. Average Atomic mass - weighted average mass of the isotopes of that element. This is the decimal number
on the periodic table.
a. average atomic mass = (Percent X Mass Number)isotope 1 + (Percent X Mass Number)isotope 2…
A. Chlorine
25% is chlorine-37
75% is chlorine – 35
What is the atomic mass of chlorine?
2. Percent Abundance--percentage of atoms in a natural sample of the pure element represented by a
particular isotope
a. percent abundance = number of atoms of a given isotope
x 100
Total number of atoms of all isotopes of that element
II. Mole
1 mole = 6.022 X 1023 particles, molecules, electrons
1 mole = 22.4 liters of gas at STP
1 mole = molar mass in grams
A’. How many molecules are in 3.00 moles of N2?
B. How many moles of Na are in 1.10 x 1023 atoms?
1. Molar Mass- is the mass (think grams) of one mole of a substance
a. Atomic masses of atoms are relative masses based on the mass of carbon-12
b. To calculate the molar mass of a compound, you add up the molar masses of all the elements
in that compound.
C. Find the molar mass of Na2SO4
2 Percent Composition aka mass composition-the percentage by mass of each element in a compound.
a.% mass = mass of 1 element x 100
mass of compound
D. What is the percent of C & H in C2H6?
3. Empirical Formula- This is the lowest whole number ratio of the elements in a compound.
To calculate the empirical formula, given the mass or percent of elements in compound, follow these steps:
a. If given a percent sign, remove the sign & change to grams. You are assuming you have 100 g
of the compound.
b. Convert the grams to moles.
c. Decide which number of moles is the lowest, then divide each number of moles by this
number.
d. If the number divides out evenly, these are the subscripts of the elements in the compound. If
any of the numbers are not a whole number, multiply them all by a factor that yields a whole
number.
E. Find the empirical formula given: 65.2% Sc and 34.8 % O
4. Molecular Formula- gives the actual # of atoms of each element in one molecule of the substance
a. You must have the the molar mass (MM) of the compound and compare it to the MM of the
the empirical formula
b. Molar Mass of compound (found by experiment) = Multiply subscripts by this number
Molar Mass of empirical formula
F. The empirical formula of a compound is CH4. It has been determined that a molecule’s MM is 32g. What is
the molecular formula?
Mixed Practice:
1. One mole of CO contains:
One mole of CO2 contains:
__________________ moles of Carbon
__________________ moles of Oxygen
__________________ moles of Carbon
__________________ moles of Oxygen
2. How many grams of carbon are contained in 55.0 g of CO2?
3. Find the percentage composition of each element in K2CO3
4. Determine the empirical formula of a compound with the following percentage composition:
85.63 % Carbon; 14.37% Hydrogen
5. If the molar mass of the above is 42.0 g, find the molecular formula.