8. CHEMICAL FORMULAE
associated with acids, bases and salts, the solubility of salts and solubility curves and
water of crystallisation
Index of GCSE level (~US grades 8-10)
notes acids, bases, salts
What next?
Associated Pages
Summary
This page tabulates many important formulae of
oxides, hydroxides, carbonates, and the salts chlorides, sulfates/sulfates, and
nitrates. Solubility data for selected salts is given together with their
solubility curves graphical representation and how to do simple solubility graph
reading and mass of salt crystallising calculations. Finally how to
calculate the water of
crystallisation from heating the salt i.e. from mass loss data. These
revision notes, data tables and solubility curve graphs of various compounds
including salts, should prove useful for the new AQA chemistry, Edexcel
chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.
Doc Brown's
chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level
& ~US grades 8, 9, 10 school science courses for ~14-16 year old science
students for national examinations in chemistry topics including acids
bases alkalis salts preparations reactions
8a. A
Summary of important formulae, solubility and water of crystallisation
For GCSE students see
the 8b. solubility summary - its all you need!
Other than the oxides,
hydroxides and carbonates (1st column), the original acids
are
hydrochloric acid HCl, sulfuric acid H2SO4
and nitric acid HNO3
which give the salts (2nd column) when reacted
with a metal, oxide, hydroxide or carbonate.
Note that
salt preparations are described in section 6
The negative ions (anions):
O2–
oxide (O), OH– hydroxide (OH),
CO32– carbonate
(CO3), HCO3– hydrogencarbonate (HCO3),
Cl– chloride (Cl), SO42– sulfate (SO4),
NO3– nitrate (NO3)
Assume the compounds are soluble
unless otherwise stated.
The 3rd right column shows what
ion/metal can be 'substituted' into the formulae in the 1st/2nd columns.
QUITE SIMPLY YOU SUBSTITUTE M with
the appropriate metal symbol TO GET THE FORMULA
The valency is the numerical
chemical combing power of an atom or an ion.
Formulae of
bases: oxides, hydroxides and carbonates
'molecular' formula and the
'real' ionic formula |
Formulae of salts formed:
soluble chlorides, sulfates and nitrates
'molecular' formula and the
'real' ionic formula |
The metal (or other ion) involved |
M2O oxide (M+)2O2–, soluble
oxides, alkali e.g. lithium
oxide Li2O
(O and S both in Group 6, so sulfides have similar formula e.g. Na2S)
MOH hydroxide M+OH–, soluble hydroxides, alkali
e.g. sodium hydroxide NaOH
M2CO3 carbonate (M+)2CO32–,
soluble carbonates, mild alkalis
e.g. sodium carbonate Na2CO3
MHCO3 hydrogencarbonate
M+HCO3–, soluble hydrogen carbonates,
mild alkalis
e.g. sodium hydrogencarbonate NaHCO3
|
MCl chloride, M+Cl–
e.g. sodium chloride NaCl
same formulae for fluorides, bromides and iodides
M2SO4 sulfate, (M+)2SO42–
e.g. potassium sulfate K2SO4
MNO3 nitrate, M+NO3–
e.g. lithium nitrate LiNO3
|
Valency 1
M = Li lithium, Na sodium, K potassium,
usually Group 1
for the M+ ion |
MO oxide M2+O2–, often insoluble basic
oxides (bases) e.g. magnesium
oxide MgO
(O and S both in Group 6, so sulfides have the same formula e.g.
MgS, CuS)
M(OH)2 hydroxide M2+(OH–)2, often insoluble
hydroxides, alkali
if soluble e.g. calcium hydroxide is slightly soluble.
e.g. zinc hydroxide Zn(OH)2
MCO3 carbonate M2+CO32–, often insoluble
e.g. calcium carbonate CaCO3
|
MCl2 chloride M2+(Cl–)2
e.g. zinc chloride ZnCl2
same formulae for fluorides, bromides and iodides
MSO4 sulfate* M2+SO42–
e.g. magnesium sulfate MgSO4
M(NO3)2 the nitrate M2+(NO3–)2
e.g. copper(II) nitrate Cu(NO3)2
All soluble salts but CaSO4
is
not very soluble
|
Valency 2
M = Mg magnesium, Ca calcium, Cu copper(II), Zn zinc, Fe iron(II),
usually Group 2 or Transition metal
for the
M2+ ion
|
Al2O3,
Al(OH)3 aluminium oxide and aluminium hydroxide are insoluble
amphoteric bases Fe2O3,
Fe(OH)3 iron (III) oxide and iron(III) hydroxide are insoluble |
AlCl3, Al2(SO4)3,
Al(NO3)3
aluminium chloride, aluminium sulfate and
aluminium nitrate are all
soluble salts |
Valency 3
Al3+ ion, aluminium in Group 3
Fe3+ ion, iron(III), transition
metal ion |
the
alkaline very soluble base ammonia, NH3, no
stable hydroxide i.e. NH4OH doesn't exist |
NH4Cl,
(NH4)2SO4, NH4NO3
ammonium chloride, ammonium sulfate and
ammonium nitrate are all
soluble salts |
the
ammonium ion, NH4+, in the salts from ammonia,
it has a valency of 1 |
How
to work out formulae is explained on another web page
Note that
salt preparations are described in section 6
8b
Solubility of salts and solubility curves (brief guide for GCSE students)
A
solubility guide for salts - information required to
decide on the method used to prepare a salt |
salts |
solubility? |
common salts of sodium,
potassium and ammonium ions |
usually soluble in water |
common sulfates (sulfates) |
usually quite soluble except for
calcium sulfate (slightly soluble), lead sulfate and barium
sulfate are both insoluble |
common chlorides (similar rule
for bromides and iodides) |
usually soluble except for
insoluble lead(II) chloride and silver chloride |
common nitrates |
all soluble |
common carbonates |
most metal carbonates are
insoluble apart from sodium, potassium and ammonium carbonate
which are soluble. |
8c. contd. Examples of Solubility Curves

-
Interpretation of graph eg
-
Reading graph: at 38oC the
solubility of copper sulfate, CuSO4, is 28g of anhydrous
salt per 100g of water.
-
Reading graph: at 84oC the
solubility of potassium sulfate, K2SO4, is 22g
per 100g of water.
-
Ex Q1: How much potassium
nitrate will dissolve in 20g of water at 34oC?
-
At 34oC the
solubility is 52g per 100g of water,
-
so scaling down, 52 x
20 / 100 = 10.4g will dissolve in 20g of water
-
Ex Q2: At 25oC
6.9g of copper sulfate dissolved in 30g of water, what is its
solubility in g/100cm3 of water?
-
Ex Q3: 200 cm3
of saturated copper solution was prepared at a temperature of 90oC.
What mass of copper sulfate crystals form if the solution was cooled to
20oC?
-
Solubility of copper
sulfate at 90oC is 67g/100g water, and 21g/100g water at 20oC.
-
Therefore for mass
of crystals formed = 67 – 21 = 46g (for 100 cm3 of solution).
-
However, 200 cm3 of
solution was prepared,
-
so total mass
of copper sulfate crystallised = 2 x 46 = 92g
Note: The density of
water is close to 1.0g/cm3 or ml, so for approximate purposes.
the volume in cm3 or ml of just the water is numerically close to
the value in g, i.e. 100 cm3 of water or solution is about 100g
of water.
Examples of |
SALT SOLUBILITY DATA in
g salt / 100g water |
Salt name |
potassium
nitrate |
potassium
sulfate |
sodium
chloride |
copper sulfate |
and formula |
Temp. oC |
KNO3 |
K2SO4 |
NaCl |
CuSO4
(anhydrous *) |
0 |
13.9 |
7.4 |
35.7 |
14.3 |
10 |
21.2 |
9.3 |
35.8 |
17.4 |
20 |
31.6 |
11.1 |
36.0 |
20.7 |
30 |
45.3 |
13.0 |
36.2 |
24.2 |
40 |
61.4 |
14.8 |
36.5 |
28.7 |
50 |
83.5 |
16.5 |
36.8 |
33.8 |
60 |
106.0 |
18.2 |
37.3 |
40.0 |
70 |
|
19.8 |
37.6 |
47.0 |
80 |
|
21.4 |
38.1 |
56.0 |
90 |
|
22.9 |
38.6 |
67.5 |
100 |
|
24.1 |
39.2 |
80.0 |
|
|
|
|
* multiply by 1.562 for hydrated crystals CuSO4.5H2O |
8c.
Water of crystallisation calculations
-
How do we determine the water of crystallisation
of a salt?
-
Given data, how do calculate the water of
crystallisation of a salt?
-
Determination and
calculation of salt formula containing 'water of
crystallisation'.
-
Some salts,
when crystallised from aqueous solution, incorporate water molecules
into the structure. This is known as 'water of crystallisation', and the
'hydrated' form of the compound.
-
e.g. magnesium sulfate MgSO4.7H2O.
The formula can be determined by a simple experiment (see the copper
sulfate example below).
-
A known mass of the hydrated salt is gently
heated in a crucible until no further water is driven off and the weight
remains constant despite further heating. The mass of the anhydrous salt left
is measured.
The original mass of hydrated salt and the mass of the anhydrous salt
residue can be worked out from the various weighings.
-
The % water of
crystallisation and the formula of the salt are calculated as follows:
-
Suppose 6.25g of blue
hydrated copper(II) sulfate, CuSO4.xH2O, (x
unknown) was
gently heated in a crucible until the mass remaining was 4.00g. This
is the white anhydrous copper(II) sulfate.
-
The mass of anhydrous
salt = 4.00g, mass of water (of crystallisation) driven off =
6.25–4.00 = 2.25g
-
The % water of
crystallisation in the crystals is 2.25 x 100 / 6.25 = 36%
-
[ Ar's
Cu=64, S=32, O=16, H=1 ]
-
The mass ratio of CuSO4
: H2O is 4.00 : 2.25
-
To convert from mass
ratio to mole ratio, you divide by the molecular mass of each
'species'
-
CuSO4 = 64
+ 32 + (4x18) = 160 and H2O = 1+1+16 = 18
-
The mole ratio of CuSO4
: H2O is 4.00/160 : 2.25/18
-
which is 0.025 : 0.125
or 1 : 5, so the formula of the hydrated salt is CuSO4.5H2O
All concentration calculations are covered on the
on–line
calculations page, especially sections 7. on molarity, 11. and 12. on molarity and
acid–base (alkali) titrations, section 14.3 on dilutions.
Note that
salt preparations are described in section 6
What next?
Associated Pages
Index of all my GCSE level (~US grades
810) notes on acids, bases
and salts
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GCSE/IGCSE Acids & Alkalis revision notes sub–index:
Index of all pH, Acids, Alkalis, Salts Notes 1.
Examples of everyday acids, alkalis, salts, pH of
solution, hazard warning signs : 2.
pH scale, indicators, ionic theory of acids–alkali neutralisation : 4.
Reactions of acids with
metals/oxides/hydroxides/carbonates, neutralisation reactions : 5.
Reactions of bases–alkalis
like ammonia & sodium hydroxide : 6. Four methods
of making salts : 7. Changes in pH in a
neutralisation, choice and use of indicators : 8. Important formulae
of compounds, salt solubility and water of crystallisation :
10.
More on Acid–Base Theory and Weak and Strong Acids
See also
Advanced Level Chemistry Students Acid–Base Revision
Notes – use index
GCSE/IGCSE foundation–easier multiple choice quiz on pH, Indicators, Acids,
Bases, Neutralisation and Salts
GCSE/IGCSE higher–harder multiple choice quiz on pH, Indicators, Acids,
Bases, Neutralisation and Salts
GCSE/IGCSE Structured question worksheet on Acid
Reaction word equations and
symbol
equation questions
Word
equation answers and
symbol
equation answers)
GCSE/IGCSE word–fill worksheet on Acids,
Bases, Neutralisation and Salts
GCSE/IGCSE
matching pair quiz on Acids, Bases, Salts and pH
See also
Advanced Level Chemistry Students Acid–Base Revision
Notes – use index
|