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Chapter 3/7 Notes – Chemical Quantities
Honors Chemistry
(student edition)
Chapter 3/7 HW- see handout!
good figures in the book to look at....
all the figures have something to offer, especially
figure 4 which is a graphic organizer comparable to our “mole map”. The sample problems in this chapter will be
especially helpful since they show all the work involved in a step-by-step fashion.
3.3
Counting Atoms
Stoichiometry :
Formula Mass : The sum of the atomic masses of all atoms(atomic masses) in a formula.
When calculating the atomic masses, round the periodic table mass to the nearest tenth!
Atomic masses are measured in
.
Examples:
H2
H2O
Ca(OH)2
Molecular mass - same thing, but truly used for
only.
Gram Atomic Mass/Gram Formula Mass: Same thing as above, but the unit is
ex - H2O
.
formula mass (or molecular mass) =
gram formula mass
=
(also called gram molecular mass - also called gram molecular weight)
The Mole: The amount of a compound or element that contains
particles of that substance.
Atoms are tiny, so we count them in “
A mole is a “
1 mole = 1 gram formula mass =
Element
Na
O2
HCl
NaCl
7.3
Moles
1
1
1
1
Atoms/Molecules/Formula Units
6.02 x 1023 atoms
6.02 x 1023 molecules
6.02 x 1023 molecules
6.02 x 1023 formula units
”.
”.
particles
Mass
grams
grams
grams
grams
Mole-Mass and Mole-Volume Relationships
1
Mole Relationships
For starters in chemistry, we have to be able to convert between moles, grams, and
molecules/atoms of substance (also liters when we work with gases)
use the “mole map”
_____ mole
_____ atom/molecule
_____ gram
_____ liter (at STP)
Oh yeah… These are dimensional analysis problems (factor label method). As far as sig figs go,
what you start with, you end with!
2 steppers
convert 13.8 g Li to moles
convert 2.0 moles Ne to g
convert 3.0 moles of Be to atoms
convert 44.8 L of O2 to moles
3 and 4 steppers
convert 128 g of O2 to molecules of O2
convert 128 g of O2 to atoms of oxygen
if there are 9.71 x 1024 atoms of oxygen in a sample of SO3, how many grams of SO3? liters?
Percent Composition and Chemical Formulas
Percentage Composition - every compound has a certain percentage of each type of atom
2
(we measure it by
)
% composition =
ex 1 - calculate % composition if a compound contains 24 g of Carbon and 64 g of Oxygen
ex 2 - what is the percentage composition of Ba(OH)2
7.4 Determining a compound’s empirical and molecular formula
empirical formula - the lowest whole number ratio of atoms in a compound (simplest formula)
3 basic types of problems - first, two rules
1. Divide (%’s or grams) by the gram atomic mass
2. Divide the resulting #’s by the smallest of those numbers
3. Multiply by 2 or 3 only if a whole number ratio isn’t the result of step 2
example 1 - calculate the empirical formula if there is 52.17 % C, 13.04% H, and 34.78 % O
example 2 - calculate the empirical formula is there is 26.56 % K, 35.41 % Cr, and 38.03 % O
3
example 3 - Find the empirical formula if a sample contains 5.6 g N and 12.8 g O
sometimes , the information provided isn’t always the same...
example 4 - an unknown compound decomposes into 1.134 g of I2 (s), and 100.0 cm3 H2(g)
Finding Molecular Formula - the same steps as empirical formula with one additional
step - use the gram formula mass of the empirical formula and its relationship to the gram
formula mass of the molecular formula to find what number to multiply the empirical formula by
to find the molecular formula (sounds more complicated than it is)
example 5 - Find the molecular formula of a compound (mm = 144.0 g) with 66.67 % C,
11.11 % H, and 22.22 % O
4