Ch 7
Writing chemical compounds
Ionic Compounds
Formula unit – it is the formula for an ionic compound.
When writing an ionic compound, we must get a neutral compounds.
(charges must balance out)
Cation
Anion
Na+
Mg+2
Al +3
Cl Cl Cl -
Na+
Mg+2
Al +3
O-2
O-2
O-2
Compound
Charges of “d” block metals
 “d” block elements can have multiple charges. See pg 205 for more,
but the following elements you need to know.
Zinc
Silver
Ag+
Copper (I) Cu+
Zn+2
Chromium (II) Cr+2
Chromium (III) Cr+3
Copper (II)
Cu+2
Iron (II)
Fe+2
Lead (II)
Pb+2
Lead (IV) Pb+4
Tin (II)
Sn+2
Tin (IV)
Iron (III)
Fe+3
Sn+4
Naming Binary Compounds – contains two different elements,
one metal and one non-metal.
Metal name
Compound Formula
NaCl
MgO
K2S
AlCl3
non-metal root + ide
Compound Name
When using a “d” block metal that has several possible ions, we use the
Roman numeral to specify which one we are using.
Compound Formula
FeCl2
FeCl3
CuBr
CuBr2
Compound Name
Compounds using the polyatomic ions use:
Metal name
Cation
Na+
K+
Anion
PO4-3
SO3-2
NH4+
Cu+2
SO4-2
CO3-2
Polyatomic ion name
Compound
Name
Remember, there are several polyatomic ions that are similar. They are
actually a series.
If there are 2 in the series:
_____ ate (greatest # of O2)
_____ ite
If there are 4 in the series:
per _____ ate (greatest # of O)
_____ ate
_____ ite
hypo _____ ite
Covalent Compounds
Binary Molecular Compounds – contains 2 non-metals.
Num. Prefix + element name
Number
Numerical Prefix
1
mono
2
di
3
tri
4
tetra
5
penta
6
hexa
7
hepta
8
octa
9
nona
10
deca
Num. Prefix + root name + ide
** Exception:
Don’t use “mono” on the
first term.
Molecular
Compound
Molecular Name
NO2
CO
N2O5
PCl3
SO2
P4S3
** Exception: When hydrogen is the 1st element in the compound, there
are no prefixes at all.
Ex. HCl
H2S
hydrogen chloride
hydrogen sulfide
These are gases and we will rarely see in this form.
Most generally, if H is the first element in a compound, it is an acid.
Acids have a sour taste (although this is not used in lab situations)
There are two types of acids:
1.
Monoxoacids – water solutions of molecular compounds composed
of H and a non-metal other than O and C.
Hydro + root + ic
Ex. HCl
H 2S
Acid
hydrochloric acid
hydrosulfic acid
2.
Oxoacids – water solutions of molecular compounds composed of H
and some non-metal and O. (normally a polyatomic ion)
Polyatomic root + “ic” or “ous”
Ex. HNO3
H2CO3
H2SO4
HClO2
HClO
Acid
nitric acid
carbonic acid
sulfuric acid
chlorous acid
hypochlorous acid
Ex. Write the formula for:
a) Sulfurous acid
(came from sulfite)
H2SO3
b) Hydrobromic acid
(hydro means it was a binary acid)
HBr
c) Chloric acid
(came from chlorate)
HClO3
** See Handout
Salt – ionic compound composed of a cation and the anion from an
acid.
acid + base
HCl + NaOH
H2SO4 + Ca(OH)2
salt + H2O
NaCl + H2O
CaSO4 + H2O
Sec 7-2
 Skipping
Sec 7-3
Formula mass – mass of any molecule, formula unit, or ion. It is the
sum of the masses of each element in the formula.
Ex. H2O
1 H atom has a mass of 1.01 amu x 2 = 2.02
1 O atom has a mass of 16.00 amu = 16.00
18.02 amu
Ex. HNO3
63.02 amu
Ex. Na2SO4
142.04 amu
Molar mass – mass in grams of 1 mole of any molecule, formula unit,
or ion.
Ex. H2O
18.02 g for every mole
ex. Ba(NO3)2
261.35 g for every mole
Conversions:
Ex. What is the mass in grams of 2.50 mol of O2?
Ex. What is the mass in grams of 3.04 mol of Ammonia vapor, NH3?
Ex. Calc the mass of .257 mol of calcium nitrate.
Ex. How many moles are there in 6.60 g (NH4)2SO4?
Ex. How many moles are there in 4.5 kg Ca(OH)2 ?
Ex. How many molecules are there in 25.0 g H2SO4?
Ex. How many formula units are there in 7.95 g CaBr2?
Ex. What is the mass in grams of 6.25 mol of Copper (II) Nitrate?
Ex. Ibuprofen C13 H18 O2 is an active ingredient in many pain
relievers.
a) How many molecules are in each tablet?
b) What is the mass in grams of Carbon in 33g of Ibuprofen?
% Composition - % by mass of each element in a compound.
Ex. Cu2S
Cu =
= 79.85 %
S =
= 20.15 %
Ex. Calculate the percent composition of NaNO3.
Na =
= 27.05%
N =
= 16.48%
O =
= 56.47%
Some salts crystallize from a water solution. They bind with water
molecules in their crystalline structure and is called a hydrate.
Sodium carbonate forms such a hydrate , in which 10 molecules of
H2O are present for every formula unit of sodium carbonate. It is
written as Na2CO3 ∙ 10 H2O.
Ex. Find the % composition of water in sodium carbonate
decahydrate.
H2O =
= 62.94 % of the compound is water
7-4
When a new substance is discovered or synthesized, it is analyzed
quantitatively to reveal its % composition. From that, its empirical
formula can be determined.
An empirical formula consists of the symbol for the elements
combined in a compound with subscripts showing the smallest whole
# mole ratio of the different atoms in the compound.
Ex. N2O4 (molecular formula)
NO2 (emp. form)
Ex. C6H12O6 (mol. form)
CH2O (emp. form)
Ex. CHCl3 (molecular)
CHCl3 (emp.) can’t be reduced
Ex. Quantitative analysis shows that a compound contains 32.38% Na,
22.65% S, and 44.99 % O. Find the empirical formula.
Ex. Repeat but with 63.52 % iron and 36.48% S.
Analysis of a 10.15 g sample of a compound known only to contain
phosphorous and oxygen indicates a phosphorous content of 4.433 g.
What is the empirical formula?
Molecular formula = whole # x empirical formula
Recall: C6H12O6 (mol. form)
CH2O (emp. form)
The whole # that would be
multiplied is 6.
whole # =
Ex. Earlier, we found the empirical formula P2O5. Experimentation
shows that the molar mass is 283.89 g/mole. Find the molecular
formula.
whole # =
=2
Therefore the molecular formula is P4O10 .
Acetylene has a % composition of 92.24% C, 7.742% H. Its
molecular mass is 26.04 g. Write both the emp. and molecular
formulas.