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Chemistry 11
Chemical Nomenclature
Formulae of the Elements
a. all noble gases are monatomic (one atom present)
ex. He, Ne, Ar. Kr, Xe, and Rn
b. the halogens and remaining gases are diatomic (two atoms present)
ex. H2, O2, F2, Br2, I2, N2, Cl2 (HOFBrINCl the Beer Baron!!!)
c. two elements are polyatomic (more than two atoms present)
ex. sulphur is S8, and white phosphorus is P4
d. all other elements are represented simply by their symbol
Oxidation Numbers
Chemists use oxidation numbers to keep track of the positive or negative character of
atoms or ions. When electrons are removed completely or shifted partially away from
an atom during a chemical reaction, the atom is given a more positive oxidation
number. When electrons are gained or shifted toward an atom during a chemical
reaction, the atom is given a more negative oxidation number. The actual numerical
value of the oxidation number depends on the number of electrons shifted partially or
transferred completely.
The oxidation number of elements is always equal to zero.
ex.
oxidation numbers:
H2 + O2 
0
0
The oxidation number of an ion is always equal to the charge on the ion.
ex.
oxidation numbers:
Na1+ + Cl1- 
+1
-1
The oxidation number of an element in a polar covalent compound is the charge that
would result if all partially transferres electrons were completely transferred to the more
electronegative element.
ex.
H 2O
each H has a partial 1+ charge, and the O has a partial 2- charge
oxidation numbers:
+1 -2
The general trend in oxidation numbers is:
group number:
I
oxidation number:
II
III
IV
V
+1
+2
+3 +4/-4
VI
VII
-3
-2
-1
The oxidation numbers of the elements found outside these groups do not follow this
order, and must be in some cases memorized. In addition, some of the elements found
inside these groups may not follow the trend, or have more that one possible oxidation
number.
The oxidation numbers in this table must be memorized.
oxidation number (+)
1
2
1
1
2
2
2
2
2
2
2
2
2
3
4
5
6
names of elements
copper – cuprum
mercury
iron – ferrum
cobalt
nickel
lead – plumbum
tin – stannum
manganese
chromium
antimony
3
3
3
4
4
4
3
3
these two only have one oxidation number but you have to memorize them!
silver
zinc
1
2
6
5
Calculating Oxidation Numbers
Whenever a compound forms there will always be a positive component and a negative
component. The positive component is always written first in both the name and the
formula of the compound.
ex.
Na1+ + Cl1-  NaCl
C + 02  CO2
sodium chloride
carbon dioxide
Unless a molecule is written as an ion. it must be electrically neutral. That is, within
the molecule we have:
total POSITVE charges = total NEGATIVE charges
This means that all of the oxidation numbers present in a neutral compound must add up
to equal zero.
NaCl
sodium is in group I and will have an oxidation # of +1
chlorine is in group VII and will have an oxidation # of –1
1 x Na = 1 x (+1) = +1
1 x Cl = 1 x (-1) = -1
add to equal
0
CO2
carbon is in group IV and will have an oxidation # of +4
chlorine is in group VI and will have an oxidation # of –2
1 x C = 1 x (+4) = +4
2 x O = 2 x (-2) = -4
add to equal
0
V2O5
vanadium is a transition metal and has an unknown ox. #
oxygen is in group VI and will have an oxidation # of –2
2 x V = 2 x (X ) = 2X
5 x O = 5 x (-2) = -10
add to equal
0
solve for x: 2x + (-10) = 0
2x –10 = 0
therefore the oxidation number for vanadium is +5
Problem Set #1
Calculate the oxidation numbers for each element in the following examples:
a. Zn2+
b. Cu2O
c. SrS
d. MgH2
e. CaO
f. CO2
g. Fe2O3
h. AlF3
i. Li2O2 (tricky)
In addition:
the oxidation number for hydrogen is normally +1, but it can also be –1 in case of
the metal hydrides (a metal hydride is a compound which has a metal atom
bonded to hydrogen with a negative oxidation number).
ex.
NaH
sodium is in group 1, and will have an oxidation # of +1
hydrogen is written last so it has a negative oxidation # (-1)
the oxidation number for oxygen is normally –2, but it can also be –1 in the case
of peroxides (a peroxide is a compound which has a metal atom bonded to one or
more oxygen atom(s) than usual).
ex.
Na2O
NA2O2
sodium oxide
sodium peroxide
Naming Compounds Containing Two Elements
Compounds that contain only two elements are called binary compounds. There are
three types of binary compounds that we will study:
1. binary compounds containing one metal and one nonmetal
2. binary compounds containing two nonmetals
3. binary acids
Lesson 2
One Metal and one Nonmetal
The system for naming these compounds is to name the metal first and then the
nonmetal with an ide ending.
ex.
NaCl
sodium chloride
- the metal has been named first and the nonmetal has had the
last few letters dropped in favour of the ide ending
- note that they are two separate words (they are not joined)
CaO
calcium oxide
Try these:
a. Al2O3
________________________
b. Ag2S
________________________
c. CaCl2
________________________
d. MgO
________________________
This sytem however, runs into trouble when dealing with metals which have multiple
oxidation numbers.
ex.
FeO
Fe2O3
this compound is named iron oxide
this compound is also iron oxide
We can distinguish between these two compounds if we mention the oxidation number
of the metal.
ex.
FeO
Fe2O3
this compound is named iron (II) oxide
this compound is named iron (III) oxide
This system of naming binary compounds containg metals with multiple oxidation
numbers is called the Stock System. When naming compounds with the Stock System,
the oxidation number of the metal is listed in Roman numerals and is enclosed within a
set of brackets.
This system is to be used only in the case of metals with multiple oxidation
numbers. Listing the oxidation number of a metal that has only one possible oxidation
number would be a mistake and the name would be incorrect.
ex.
CuO
CaO
copper (II) oxide
calcium (II) oxide
Try these:
a. FeCl2
________________________
b. CrO3
________________________
c. NiF2
________________________
d. Sb2S3
________________________
CORRECT
INCORRECT
There is also an older method of naming compounds containing metals with multiple
oxidation numbers. This system (affectionately called the “ous” and “ic” system) uses
the Latin names for the metals and special endings to distinguish the compounds. The
endings are:
ous to signify the lower oxidation number (o for low)
ic
to signify the higher oxidation number (I for high)
NOTE: this system will only work for those metals with two possible oxidation
numbers.
ex.
Cu2O
CuO
FeO
Fe2O3
PbO
PbO2
SnO
SnO2
copper (I) oxide
copper (II) oxide
iron (II) oxide
iron (IV) oxide
lead (II) oxide
lead (IV) oxide
tin (II) oxide
tin (IV) oxide
Try these:
a. SnF2
________________________
b. PbS2
________________________
cuprous oxide
cupric oxide
ferrous oxide
ferric oxide
plumbous oxide
plumbic oxide
stannous oxide
stannic oxide
To obtain the formula of a binary compound with 1 metal and 1 nonmetal:
a. write the symbol for each element, putting the metal atom first
b. write the oxidation numbers as superscript to the right of each element
c. check if both oxidation numbers can be reduced by a common factor
d. switch the oxidation numbers for the elements and write them as subscripts
ex.
magnesium fluoride
Mg2+ F1-
switcheroo!
Mg1 F2
simplify
MgF2
aluminium oxide
Al3+ O2-
switcheroo!
Al2
simplify
Al2O3
barium sulfide
Ba2+ S2-
reduce the numbers
Ba1+ S1-
switcheroo!
Ba1
simplify
BaS
Try these:
a. calcium sulphide
O3
S1
(only the numbers not the charges)
(only the numbers not the charges)
(only the numbers not the charges)
(Ba2O2 would be incorrect)
________________________
b. aluminium carbide
________________________
c. zinc arsenide
________________________
d. barium nitride
________________________
Problem Set #2: Binary Compounds / One Metal and One Nonmetal
Name each of the following. Where applicable, give both Stock and ous/ic names.
a. ZnS
b. FeO
c. Ag2O
d. SbCl3
e. CuI2
f. PbO2
g. NiN
h. MnO2
i. BeH2
j. Bi2S3
k. BaO
l. LiCl
Lesson 3
Two NonMetals
These compounds are named by IGNORING ALL OXIDATION NUMBERS and
simply using the prefix system to identify how many of each element is present in the
compound. (you must memorize these)
1 = mono
4 = tetra
7 = hepta
2 = di
5 = penta
8 = octa
3 = tri
6 = hexa
9 = nona
10 = deca
NOTE: in cases where there is only one atom of the first element do not use the prefix
mono – instead just list the name of the element.
The elements are always written in the order given and the last element is given an ide
ending.
ex.
CO
carbon monoxide
- the carbon has been named first and the oxygen has had the
last few letters dropped in favour of the ide ending
- note that they are two separate words
N2O4
dinitrogen tetraoxide
Try these:
a. P2O3
________________________
b. SO3
________________________
c. CCl4
________________________
d. N2O3
________________________
To obtain the formula of a binary compound with two nonmetals simply write the
symbol for each element in the order given and include any necessary prefixes.
ex.
diphosphorous pentaoxide
P2O5
sulphur hexafluoride
SF6
Problem Set #3: Binary Compounds / Two Nonmetals
Name each of the following compounds using the prefix system:
a. PF3
b. NO2
c. AsBr5
d. SO2
e. SiBr4
f. ICl
Give the formula for each of the following compounds:
g. nitrogen triiodide
h. sulphur dichloride
i. arsenic pentafluoride
j. silicon tetrachloride
k. nitrogen monoxide
l. tetraphosphorous decaoxide
Problem Set #4: Binary Compounds
Name each of the following compounds:
a. KH
b. B2O3
c. Na2O
d. HgCl2
e. PbF2
f. CoO
g. BeI2
h. LiF
i. Sb2O5
Give the formula for each of the following compounds:
j. ferrous oxide
k. phosphorous pentiodide
l. tin (II) sulphide
m. strontium iodide
n. cupric fluoride
o. silver chloride
p. radium nitride
q. magnesium bromide
r. dinitrogen trioxide
Lesson Four
Binary Acids
The formulae for binary acids, of which there are only five, are very easy to recognize.
They will always contain hydrogen and one other element and will always be followed
by an (aq). This symbol (aq) means the material is an aqueous mixture (dissolved in
water) and the compound should be named as an acid. If the (aq) is absent the
compound should not be named as an acid.
All of the binary acids are named in the same way. They all have a hydro prefix and an
ic ending. In this case the name is written as one word.
The five binary acids are:
1. HF (aq)
hydrofluorous acid
2. HCl (aq)
hydrochloric acid
3. Hbr (aq)
hydrobromic acid
4. HI (aq)
hydroiodic acid
5. H2S (aq)
hydrosulphuric acid
Naming Compounds Containing More Than Two Elements
Compounds that contain more than two elements are called polyatomic compounds.
There are two types of polyatomic compounds that we will be studying:
1. polyatomic or oxy acids
2. polyatomic compounds or salts
Lesson Five
The Oxy Acids
These compounds are very easy to recognize. They will always contain hydrogen,
oxygen and one other element, and will always be followed by an (aq). If the (aq) is
absent the compound should not be named as an acid.
There are many oxy acids but they are all based upon the formulae of eight acids. The
names and formulae of these eight acids must be memorized before any sense can be
made of the rest of the acids. The acids are named according to the number of oxygen
atoms present in the formula. As the number of oxygens changes, the name also
changes.
The naming of these acids is based upon the 8 acids in the third column. All of these
acids will have the ending ic acid (ex. chloric acid). As you move to the right 1 oxygen
has been added to the formula and the name changed to per…ic acid (ex. perchloric
acid). As you move to the left 1 oxygen has been removed from the formula and the
ending changed to ous acid (ex. chlorous acid). The column to the far left has had 2
oxygens removed form the formula and the name changed to hypo…ous acid (ex.
hypochlorous acid).
see chart 1 attached
Problem Set #5: Binary and Oxy Acids
Name each of the following acids:
a. HBrO3 (aq)
b. H2CO3 (aq)
c. HBrO4 (aq)
d. H3PO3 (aq)
e. HBr (aq)
f. HClO4 (aq)
g. H2SO4 (aq)
h. HIO3 (aq)
i. HNO3 (aq)
Give the formula for each of the following acids:
j. hydrobromic acid
k. hydrosulphuric acid
l. hydroiodic acid
m. chlorous acid
n. acetic acid
o. periodic acid
p. perchloric acid
q. hypocarbonous acid
r. phosphorous acid
Lesson 6
Polyatomic Compounds / The Salts
As you saw in the case of the oxy acids not all compounds are binary. Those
compounds which contain more than two elements are called polyatomic compounds
and are named in much the same way we have named the binary compounds. The
positive portion is always named first – being careful to use the Stock or ous/ic system
if necessary – and then the negative portion is named. The only trouble is that both
positive and negative sections may be polyatomic ions, or radicals.
Radicals are a number of atoms which act and are named as a single ion.
ex.
NO31nitrate ion
PO43phosphate ion
CO32carbonate ion
Most of the radicals are based on the oxy acids and are formed when some or all of the
hydrogens on the acid are removed. Once the hydrogens are removed, the ion is left
with a negative charge equal to the number of hydrogens lost.
ex.
HClO3 (aq)
if 1 hydrogen is lost will form
ClO31-
H2SO4 (aq)
if 2 hydrogens are lost will form
SO42-
H3PO4 (aq)
if 3 hydrogens are lost will form
PO43-
These negatively charged radials are named by simply replacing the ending they had
when they were oxy acids with either and ate or an ite. All of the ic endings are
replaced with ate endings, and all of the ous endings are replaced with ite endings.
ex.
HClO3 (aq)
chloric acid
replace the ic ending with ate
ClO31- (aq)
chlorate ion
ex.
HClO2 (aq)
chlorous acid
replace the ous ending with ite
ClO21- (aq)
chlorite ion
ex.
HClO4 (aq)
perchloric acid
replace the ic ending with ate
see chart 2 attached
ClO41- (aq)
perchlorate ion
In addition to the table attached, you must also memorize the following radicals:
CrO42chromate ion
Cr2O72dichromate ion
CN1cyanide ion
NH41+
ammonium ion
OH1hydroxide ion
MnO42manganate ion
MnO41permanganate ion
Problem Set # 6: Polyatomic Compounds - Salts
Name each of the following compounds:
a. Pb(CN)2
b. Fe(OH)3
c. K2SO4
d. Fe(NO3)3
e. NaNO3
f. NH4F
g. Hg(IO3)2
h. Zn(BrO3)2
i. Ca3(PO4)2
j. NH4CH3COO
k. Sr(NO3)2
l. Cr2(SO3)3
Give the formula for each of the following compounds:
m. silver nitrate
n. barium acetate
o. potassium cyanide
p. tin (IV) iodate
q. nickel (II) nitrate
r. barium perchlorate
s. potassium permanganate t. magnesium hydroxide
u. aluminium sulphate
v. ammonium sulphite
x. ammonium hydroxide
w. cobalt (II) phosphite
Lesson 7
Polyatomic Compounds Continued / The Addition of Hydrogen
In lesson six the radicals based on the oxy acids were formed when all of the hydrogens
were removed. These acids however, hang on to one or two of their hydrogens. This
will still create a radical whose charge will be equal to the total number of hydrogens
lost, but it will change the name a little.
The name for these radicals combines the prefix system and th normal name for the
radical.
ex.
H2CO3 (aq)
carbonic acid
2 hydrogens could be removed
H2CO3 (aq)
carbonic acid
1 hydrogen could be removed
CO32carbonate ion
HCO32hydrogen carbonate ion
The charge on the radical equals the number of hydrogens lost and the name indicates
the number of hydrogens remaining and the radical present.
ex.
PO43phosphate ion
H3PO4 (aq)
phosphoric acid
3 hydrogens could be removed
H3PO4 (aq)
phosphoric acid
2 hydrogens could be removed
HPO42hydrogen phosphate ion
H3PO4 (aq)
phosphoric acid
1 hydrogen could be removed
H2PO41dihydrogen phosphate ion
Try these:
a. LiH2PO4
________________________
b. KHSO4
________________________
c. CaHPO4
________________________
d. LiHCO3
________________________
Lesson 8
Hydrate Compounds
There are many compounds that crystallize from a water solution with water molecules
remaining attached to the ions and become part of the crystal. These hydrates, as they
are called, usually contain a specific ratio of water to compound. Formulae for hydrated
compounds place the water of hydration following a dot after the regular formula.
ex.
CuSO4  5H2O
- this compound contains five moles of water
for each mole of copper (II) sulphate
- its name is copper (II) sulphate penthydrate
CaSO4  2H2O
- this compound contains two moles of water
for each mole of calcium sulphate
- its name is calcium sulphate dihydrate
Try these:
a. MgSO3  6H2O
________________________
b. BaBr2  2H2O
________________________
c. Tl(NO3)  3H2O
________________________
d. SnCl2  2H2O
________________________
Chart 1: Polyatomic Compounds / The Oxy Acids
- 2 oxygens
- 1 oxygen
hypo…ous acid
ous acid
ic acid
per…ic acid
hypochlorous
chlorous
chloric
perchloric
HClO (aq)
HClO2 (aq)
HClO3 (aq)
HClO4 (aq)
hypobromous
bromous
bromic
perbromic
HBrO2 (aq)
HBrO3 (aq)
iodous
iodic
hypoiodous
HIO (aq)
hyponitrous
+ 1 oxygen
HIO3 (aq)
nitrous
nitric
HNO2 (aq)
HNO3 (aq)
sulphurous
sulphuric
H2SO3 (aq)
H2SO4 (aq)
hypophosphorous
phosphorous
phosphoric
H3PO2 (aq)
H3PO3 (aq)
H3PO4 (aq)
hypocarbonous
carbonous
carbonic
hyposulphurous
periodic
H2CO3 (aq)
acetic
CH3COOH (aq)
pernitric
persulphuric
perphosphoric
percarbonic
Chart 2: Polyatomic Compounds / The Salts
- 2 oxygens
- 1 oxygen
hypo…ous acid
ous acid
ic acid
per…ic acid
hypochlorite
chlorite
chlorate
perchlorate
ClO1- (aq)
ClO21- (aq)
ClO31- (aq)
ClO41- (aq)
hypobromite
bromite
bromate
perbromate
BrO21- (aq)
BrO31- (aq)
iodite
iodate
hypoiodite
IO1- (aq)
hyponitrite
+ 1 oxygen
IO31- (aq)
nitrite
nitrate
NO21- (aq)
NO31- (aq)
sulphite
sulphate
SO32- (aq)
SO42- (aq)
hypophosphite
phosphite
phosphate
PO23- (aq)
PO33- (aq)
PO43- (aq)
hypocarbonite
carbonite
carbonate
hyposulphite
periodate
pernitrate
persulphate
perphosphate
percarbonate
CO32- (aq)
acetate
CH3COO1- (aq)
In addition to the table, you must also memorize the following radicals:
CrO42chromate ion
Cr2O72dichromate ion
CN1cyanide ion
NH41+
ammonium ion
OH1hydroxide ion
MnO42manganate ion
MnO41permanganate ion
Chemical Nomenclature
By following this chart the process of naming compounds should be made easier.
how many elements?
2 = binary compounds
>2 = compounds with radicals
does it have an (aq)?
does it have an (aq)?
no
binary compound
yes
= binary acid
binary acids will always have a
hydrogen and a nonmetal atom
binary acids are named with a
hydro at the beginning, the
name of the nonmetal, and an ic
ending
it should all one word and don’t
forget the acid
ex. HCL (aq)
hydrochloric acid
yes
= oxy acid
no
= salt
does it have 2 nonmetals?
yes
= prefix system
mono = 1
di = 2
tri = 4, etc.
name the most +ve atom
first
do not use mono for the
first atom
no
= 3 choices
1.
metal has 1 oxidation
number
name the metal fist then
the nonmetal with an ide
ending
ex. CaO calcium oxide
metal has >1 oxidation #
ex. CO2
carbon dioxide
2.
ex. HI (aq)
hydroiodic acid
use the Stock system
to show the oxidation
# of the metal, then the
nonmetal with an ide
ending
ex. PbO lead (II) oxide
3.
use the ous/ic system
and name the
nonmetal with an ide
ending
ex. FeO ferrous oxide
name the oxy acid
ex. HNO3 (aq)
nitric acid
name the +ve ion first being
careful to check if the Stock
or ous/ic systems should be
used and then name the –ve
ion with the correct ite or ate
ending
ex. Cu2SO4
copper (I) sulphate
cuprous sulphate
ex. (NH4)2SO4
ammonium sulphate
Answers to Problem Sets
Problem Set #1: Calculating oxidation numbers
a. Zn2+
b. Cu2O
Zn +2
Cu +1
O -2
c. SrS
Sr +2
S -2
d. MgH2
Mg +2
H -1
e. CaO
Ca +2
O -2
f. CO2
C +4
O -2
g. Fe2O3
Fe +3
O -2
h. AlF3
Al +3
F -1
i. Li2O2
Li +1
O -1
Problem Set #2: Binary Compounds / One Metal and One Nonmetal
a. ZnS
b. FeO iron (II) oxide
c. Ag2O
zinc sulphide
ferrous oxide
silver oxide
d. SbCl3
e. CuI2 copper (II) iodide
antimony (III) chloride
cupric iodide
f. PbO2 lead (IV) oxide
plumbic oxide
g. NiN
nickel (III) nitride
h. MnO2
manganese (IV) oxide
i. BeH2
beryllium hydride
j. Bi2S3
bismuth sulphide
k. BaO
barium oxide
l. LiCl
lithium chloride
Problem Set #3: Binary Compounds / Two Nonmetals
a. PF3
b. NO2
phosphorus trifluoride
nitrogen dioxide
c. AsBr5
arsenic pentabromide
d. SO2
sulphur dioxide
e. SiBr4
silicon tetrabromide
f. ICl
g. nitrogen triiodide
NO3
h. sulphur dichloride
SCl2
i. arsenic pentafluoride
AsF5
j. silicon tetrachloride
SiCl4
k. nitrogen monoxide
NO
l. tetraphosphorous decaoxide
P4O10
iodine monochloride
Problem Set #4: Binary Compounds
a. KH
b. B2O3
potassium hydride
boron oxide
c. Na2O
sodium oxide
d. HgCl2
mercury (II) chloride
e. PbF2
lead (II) fluoride
plumbous fluoride
f. CoO
cobalt (II) oxide
g. BeI2
beryllium iodide
h. LiF
lithium fluoride
i. Sb2O5
antimony (V) oxide
j. ferrous oxide
FeO
k. phosphorous pentiodide
PI5
l. tin (II) sulphide
SnS
m. strontium iodide
SrI
n. cupric fluoride
CuF2
o. silver chloride
AgCl
p. radium nitride
Ra3N2
q. magnesium bromide
MgBr2
r. dinitrogen trioxide
N2O3
Problem Set #5: Binary and Oxy Acids
a. HBrO3 (aq)
b. H2CO3 (aq)
bromic acid
carbonic acid
c. HBrO4 (aq)
perbromic acid
d. H3PO3 (aq)
phosphorous acid
e. HBr (aq)
hydrobromic acid
f. HClO4 (aq)
perchloric acid
g. H2SO4 (aq)
sulphuric acid
h. HIO3 (aq)
iodic acid
i. HNO3 (aq)
nitric acid
j. hydrobromic acid
HBr (aq)
k. hydrosulphuric acid
H2S (aq)
l. hydroiodic acid
HI (aq)
m. chlorous acid
HClO2 (aq)
n. acetic acid
CH3COOH (aq)
o. periodic acid
HIO4 (aq)
p. perchloric acid
HClO4 (aq)
q. hypocarbonous acid
H2CO (aq)
r. phosphorous acid
H3PO3 (aq)
Problem Set # 6: Polyatomic Compounds - Salts
a. Pb(CN)2
b. Fe(OH)3
lead (II) cyanide
iron (III) hydroxide
plumbous cyanide
ferric hydroxide
c. K2SO4
potassium sulphate
d. Fe(NO3)3
iron (III) nitrate
ferric nitrate
e. NaNO3
sodium nitrate
f. NH4F
ammonium fluoride
g. Hg(IO3)2
mercury (II) iodate
h. ZnBrO3
zinc bromate
i. Ca3(PO4)2
calcium phosphate
j. NH4CH3COO
ammonium acetate
k. Sr(NO3)2
strontium nitrate
l. Cr2(SO3)3
chromium (III) sulphite
m. silver nitrate
AgNO3
n. barium acetate
Ba(CH3COO)2
o. potassium cyanide
KCN
p. tin (IV) iodate
Sn(IO3)4
q. nickel (II) nitrate
Ni(NO3)2
r. barium perchlorate
Ba(ClO4)2
s. potassium permanganate t. magnesium hydroxide
KMnO4
Mg(OH)2
u. aluminium sulphate
Al2(SO4)3
v. ammonium sulphite
(NH4)2SO3
x. ammonium hydroxide
NH4OH
w. cobalt (II) phosphite
Co3(PO3)2