GCSE Chemistry: Typical common formula of acids, bases & salts

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8. CHEMICAL FORMULAE associated with acids, bases and salts,

the solubility of salts and

solubility curves and water of crystallisation

Index of all my GCSE notes on acids, bases and salts

All my GCSE Chemistry Revision notes

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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 (c) doc b 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 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

(c) doc b How to work out formulae is explained on another web page

Note that (c) doc b 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



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

solubility curves for potassium nitrate, potassium sulphate/sulfate, sodium chloride, copper(II) sulphate

  • 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?

      • Scaling up, 6.9 x 100 /30 = 23g/100g of water (check on graph, just less than 23g/100g 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




CuSO4 (anhydrous *)

























































* 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 CLICK for GCSE Chemical Calculations 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

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

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