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Brown's Chemistry
The pH scale of acidity and alkalinity,
acids, alkalis, salts and neutralisation
8. Important
formulae, solubility curves & water of crystallisation Revision
Notes KS4 Science
IGCSE/O level/GCSE
Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel
360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science
(revise courses equal to US grades 9-10)
Advanced Level Chemistry Acid-Base Revision
Notes - use index
GCSE Sub-index:
Index of all pH, Acids, Alkalis, Salts Notes 1.
Examples of acid-alkali chemistry : 2.
pH scale, indicators, ionic theory of acids-alkali neutralisation
: 3. pH examples of
acid, neutral or alkaline
solutions : 4. Acid reactions with
metals/oxides/hydroxides/carbonates and neutralisation reactions : 5.
Reactions of bases-alkalis
like sodium hydroxide : 6. Four methods
of making salts : 7. Changes in pH in a
neutralisation : 8. Important formulae, salt
solubility and water of crystallisation : 9. Further examples of word/symbol equations
for salt preparations :
10.
More on Acid-Base Theory and Weak and Strong Acids
8a. A
Summary of important formulae, solubility and water of crystallisation
Other than the oxides,
hydroxides and carbonates (1st column), the original acids
are
hydrochloric acid HCl,
sulphuric/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.
The valency is the numerical
chemical combing power of an atom/ion.
|
Formulae of
bases: oxides, hydroxides and carbonates
'molecular' formula and the
'real' ionic formula |
Formulae of salts formed:
soluble chlorides, sulphates and nitrates
'molecular' formula and the
'real' ionic formula |
The metal (or other ion) involved |
|
M2O oxide (M+)2O2-, soluble
oxides, alkali
(O and S both in Group 6, so sulfides have similar formula e.g. Na2S)
MOH hydroxide M+OH-, soluble hydroxides, alkali
M2CO3 carbonate (M+)2CO32-,
soluble carbonates, mild alkalis
MHCO3 hydrogencarbonate
M+HCO3-, soluble hydrogen carbonates,
mild alkalis
|
MCl chloride, M+Cl-
e.g. sodium chloride NaCl
M2SO4 sulphate, (M+)2SO42-
e.g. potassium sulphate 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 sulphides 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
MSO4 sulphate* 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/sulfate/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/sulphate/nitrate are all
soluble salts |
the
ammonium ion, NH4+, in the salts from ammonia,
it has a valency of 1 |
You can deduce the name from
How
to work out formulae is explained on another web page
Note that
salt preparations are described in section 6
8b
Solubility of salts - solubility curves
-
Interpretation of graph eg
-
Reading graph: at 38oC the
solubility of copper sulphate, CuSO4, is 28g of anhydrous
salt per 100g of water.
-
Reading graph: at 84oC the
solubility of potassium sulphate, 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 sulphate 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 sulphate crystals form if the solution was cooled to
20oC?
-
Solubility of copper
sulphate 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 sulphate 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 |
SOLUBILITY |
g salt / 100g water |
|
Salt name |
potassium
nitrate |
potassium
sulphate |
sodium
chloride |
hydrated
copper(II) sulphate |
|
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
-
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 sulphate MgSO4.7H2O.
The formula can be determined by a simple experiment (see the copper
sulphate 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) sulphate, CuSO4.xH2O, (x
unknown) was
gently heated in a crucible until the mass remaining was 4.00g. This
is the white anhydrous copper(II) sulphate.
-
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
-
-
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