Welcome to Learn Chemistry by Inam Jazbi, the platform where complex chemistry concepts become super-easy. Today’s post covers one of the most high-yield MDCAT topics—Solubility Product (Ksp), Solubility, Ionic Equilibria, and Precipitation Numericals. If you want to master Ksp tricks, shortcut methods, exam-focused questions, and the logic behind precipitation, this guide is made exactly for you.
Q1. Write the solubility product expression for the following slightly soluble salts; CaSO4, PbCl2 Fe(OH)3, BaF2, Li2C2O4, MgCO3, Ag3PO4.
Solution
To write Ksp expression, first of all write the balanced equation of ionic equilibrium of given compound to find out coefficient of each ion. Then Ksp expression is written as product of ionic concentrations of cation and anion of given compound enclosed in square bracket with each concertation is raised to power equal to the coefficient of respective ion.
CaSO4(s) ⇌ Ca2+(aq) + SO42−(aq) Ksp = [Ca2+] [SO42−]
PbCl2(s) ⇌ Pb2+(aq) + 2Cl−(aq) Ksp = [Pb2+] [Cl−]2
Fe(OH)3(s) ⇌ Fe3+(aq) + 3OH−(aq) Ksp = [Fe3+] [OH−]3
Q. Write down the solubility product expressions for the following sparingly soluble salts along with their units
(i) BaF2
(ii) Li2C2O4
(iii) MgCO3
(iv) Ag3PO3
(v) CaSO4
(vi) AlF3
(vii) MgCr2O7
(viii) PbI2
(ix) Mg(OH)2
(x) Mg3(PO4)2
(xi) PbCrO4
Answer
Q. Write the equation for dissociation of following salts and write down Ksp expression.
(i) PbBr2 (ii) Hg2CrO4 (iii) BaC2O4 (iv) Fe(OH)3
(v) Ag2CO3 (vi) Sb2S3 (vii) AgCNS (viii) Ag3PO4
(ix) Li3Na3(AlF6)2 (x) Hg2I2
(xi) Ba3(PO4)2 (xii) Ca5(PO4)3F
(xiii) A3B4 (xiv) CaF2
(xv) Ag2CrO4
Answer
Calculating KSP from given solubility Step-by-Step
s = molar solubility (mol L⁻¹) = moles of formula units dissolved per litre.s (and of any fixed/common ions present).mol L⁻¹ = (mass in g per L) / (molar mass in g mol⁻¹)
Common patterns (remember these!)
MX ⇌ M⁺ + X⁻ → [M] = s, [X] = s → Ksp = s²
MX₂ ⇌ M²⁺ + 2X⁻ → [M] = s, [X] = 2s → Ksp = [M][X]² = s(2s)² = 4 s³
M₂X₃ ⇌ 2M³⁺ + 3X²⁻ → [M] = 2s, [X] = 3s → Ksp = (2s)²(3s)³ = 4·27·s⁵ = 108 s⁵
(Use the stoichiometric coefficients to build the expression.)
Q2. Silver sulphate (Ag2SO4) is used for medicinal purpose to fill wounds. Its solubility in water at 25oC is 1.43 × 10–2 mol/dm3. What will be its Ksp? (Example 7.8; Page 157)
Solution
The net ionic equation for ionization
of Ag2SO4 is:
Ag2SO4(s) ⇌ 2Ag+(aq) + SO42–(aq)
1 mol 2mol
1mol
x M 2x M x M
Solubility of Ag2SO4 = x = 1.05 ×
10–5 mol/dm3
[Ag+] = 2x = 2 × 1.43
x 10–2 =
2.86 × 10–2 mol/dm3
[SO42–]
= x
= 1.43 × 10–2 mol/dm3
KSP of Ag2SO4 = ?
The KSP
expression is:
KSP = [Ag+]2 [SO42–]
KSP = (2.86 × 10–2 mol/dm3)2 x (1.43 × 10–2 mol/dm3)
KSP = 1.17
× 10–5 mol3/dm9
Q3. The Ksp of Zn(OH)2 is 2.1 × 10–16 mol3/dm9 at 25C. Calculate its solubility in g/dm3 (The atomic mass of Zn = 65.4).
Solution
KSP of Zn(OH)2 = 2.1 x 10–16 mol3/dm9
Let,
Solubility of Zn(OH)2 = x mol/dm3
The ionization equation of Zn(OH)2 is:
x mol/dm³ x mol/dm³ 2x mol/dm³
Solubility in mol/dm³
The KSP expression is:
KSP = [Zn2+] [OH–]2 ⇒ 2.1 x 10–16 = (x). (2x)2 ⇒ 2.1 x 10–16 = (x).(4x2) ⇒ 4x3 = 2.1 x 10–16
Solubility in g/dm³
Solubility of Zn(OH)2
in mol/dm³ = 3.74 × 10−6 mol/dm³
Molecular mass of Zn(OH)2 = 65.4 + 2(16) + 2(1) = 99.4 g/mol
Solubility of Zn(OH)2 in g/dm³ = Solubility in mol/dm³ × Molar mass (molecular mass)
Solubility of Zn(OH)2 in g/dm³ = 3.74 × 10−6 × 99.4
Solubility of Zn(OH)2 in g/dm³ = 3.717 × 10−4 g/dm³
Q4.Lead fluoride
(PbF2) is a high melting white solid used in glass coating to
reflect IR rays. Its solubility in water at 25℃ is 0.58 g/dm3.
Calculate its Ksp. (Atomic mass of Pb = 207 and F = 19) 5.3 × 10−8
mol3/dm9
Solution
Solubility of PbF2 = 0.58 g/dm3
Molar mass of PbF2 = 207 + 2(19) = 245g/mol
Solubility of PbF2 in mol/dm3 = x = Solubility in g/dm3/ molar mass =0.58 /245 = 2.37 × 10–3 mol/dm3
The net ionic equation for ionization
of Ag2SO4 is:
PbF2(s) ⇌ Pb2+(aq) + 2F–(aq)
1 mol 1mol 2mol
x M x M 2x M
Solubility of PbF2 = x = 2.37 × 10–3 mol/dm3
[Pb2+] = x = 2.37 ×10–3
mol/dm3
[F–] = 2x = 2 × 2.37 × 10–3 mol/dm3 =
4.74 × 10⁻³ mol/dm³
KSP of PbF2 = ?
The KSP
expression is:
KSP
= [Pb2+] [F–]2 = (2.37 × 10–3 mol/dm3) × (4.74 × 10⁻³ mol/dm3)2 = (2.37 × 10–3) × (2.25×10−5) mol3/dm9
KSP = 5.33
× 10–5 mol3/dm9
Q1. Calculate
KSP of BaSO4 whose solubility is 1.05 × 10–5
mole/dm3.
Solution
The net ionic equation for ionization
of BaSO4 is:
BaSO4 (s) ⇌ Ba2+(aq) + SO42–(aq)
1 mol 1mol
1mol
x M x M x M
Solubility of BaSO4
= x = 1.05 × 10–5 mol/dm3
[Ba2+] = x = 1.05 × 10–5 mol/dm3
[SO42–]
= x = 11.05 × 10–5 mol/dm3
KSP of BaSO4 = ?
The KSP
expression is:
KSP
= [Ag+]2 [SO42–] = (1.05
× 10–5 mol/dm3) × (1.05 × 10–5 mol/dm3)
KSP = 1.1
x 10–10 mol2/dm6
Q2. Calculate
KSP of BaF2 when its solubility is 0.0065 M at 25°C.
Solution
The ionization equation of
BaF2 is:
BaF2 ⇌ Ba2+ + 2F–
x M x
M 2x M
0.0065 M 0.0065 M 2(0.0065) M
Solubility of BaF2
= x = 0.0065 M
[Ba2+] = x = 1.05 x 10–5 M
[F–] = 2x
= 2 × 0.0065 M
KSP of BaF2 = ?
The KSP
expression is:
KSP = [Ba2+] [F–]2
KSP = (0.0065) (2 × 0.0065)2 = 0.0065 × (0.013)2 = 0.0065 × 0.000169 = 0.00000109
KSP = 1.09 × 10–6 M3
Q3. Calculate
KSP of AgCl at 25°C when its solubility is 1.4 x 10–3
g/dm3.
Solution
Solubility of AgCl = 1.4 ×
10–3 g/dm3
Molar mass of AgCl = 108 + 35.5 = 143.5 g
Solubility of AgCl in mol/dm3 = Solubility in g dm–3/ molar mass =1.4 × 10–3 /143.5 = 9.75 × 10–6 M
The ionization equation of
AgCl is:
AgCl ⇌ Ag+ + Cl–
x M x M x
M
9.75 × 10–6 M 9.75 × 10–6 M 9.75 × 10–6 M
Solubility of AgCl = x
= 9.75 × 10–6 M
[Ag+] = x = 9.75 × 10–6 M
[Cl–] = x = 9.75 × 10–6 M
KSP of AgCl = ?
The KSP
expression is:
KSP = [Ag+] [Cl–] = 9.75 × 10–6 M × 9.75 × 10–6 M2
KSP = 9.5 × 10–11 M2
Calculating Solubility from given KSP
Q1. Find the
solubility of PbCrO4 when its KSP is 2.8 × 10–13
mol2/dm6.
Solution
KSP of PbCrO4 = 2.8 x 10–13 mol2/dm6
Let,
Solubility of PbCrO4= x mol/dm3
The ionization equation of
PbCrO4 is:
PbCrO4 ⇌ Pb2+ + CrO42−
x mol/dm3 x mol/dm3 x mol/dm3
Solubility in mol/dm3
The KSP
expression is:
KSP = [Pb2+] [CrO42−] ⇒ 2.8 x 10–13 = (x).(x) ⇒ x2 = 2.8 x 10–13 ⇒ x = √2.8 x 10–13
x = 5.29 × 10–7 mol/dm3
Solubility in g/dm3
Solubility of PbCrO4
in mol/dm3= 5.29 × 10–7 mol/dm3
Molecular mass of PbCrO4 = 207 + 52 + 4(16) = 323 g/mol
Solubility of PbCrO4 in
g/dm3 = Solubility in mol/dm3
× Molar mass (molecular mass)
Solubility of PbCrO4 in g/dm3 = 5.29 × 10–7 × 323 = 1.70 × 10–4 g/dm3
Q2. Find the solubility of MgF2 when its KSP
is 7.26 × 10–9 mol3/dm9.
Solution
KSP of MgF2 = 7.26 × 10–9 mol3/dm9
Let,
Solubility of MgF2 = x mol/dm3
The ionization equation of
MgF2 is:
MgF2 ⇌ Mg2+ + 2F–
x mol/dm3 x mol/dm3 2x mol/dm3
The KSP
expression is:
KSP = [Mg2+] [F–]2 ⇒ 7.26 × 10–9 = (x). (2x)2 ⇒ 7.26 × 10–9 = (x).(4x2) ⇒ 4x3 = 7.26 × 10–9
x3 = 7.26 × 10–9/4 ⇒ x3 = 1.815
× 10–9 ⇒ x = ∛1.815 × 10–9
x = 1.219 × 10–3 mol/dm3
Here,
Solubility of MgF2
in g/dm3= 1.219 x 10–3 mol/dm3
Molecular weight of MgF2 = 24 + 38 = 62 g
Solubility of MgF2
in g/dm3 = Solubility in mol/dm3 x Molar
mass (molecular mass)
Solubility of MgF2 in g/dm3 = 1.219 × 10–3 x 62 = 7.55 × 10–2 g/dm3
Q3. What is the
ionic concentration of Ag+ and CrO42− ions in
a saturated solution of Ag2CrO4 at 25oC? Ksp
of Ag2CrO4 is 1.9 ×
10-12 mol3/dm9 (KB – 2015)
Solution
KSP of Ag₂CrO₄ = 1.9 x 10−12 mol3/dm9
Let,
Solubility of Ag₂CrO₄ = x mol/dm3
The ionization equation of Ag₂CrO₄ is:Ag₂CrO₄ ⇌ 2Ag⁺ + CrO₄²⁻
1 2 1
x mol/dm³ 2x mol/dm³ x mol/dm³
The KSP
expression is:
KSP = [Ag+]2
[CrO42–] ⇒ 1.9 x 10–12 = (x).(2x)2 ⇒ 1.9 x 10–12 = (x).(4x2) ⇒ 4x3 = 1.9 x 10–12
[Ag⁺]² = 2x = 2 × 7.8 × 10⁻⁵ = 1.56 × 10⁻⁴ moldm⁻³
[CrO₄²⁻] = x = 7.8 × 10⁻⁵ moldm⁻³
Q4. If the concentration of CrO42− ions
in a saturated solution of silver chromate is 2×10−4; calculate
solubility product of silver chromate.
Solution
The ionization equation of
Ag2CrO4 is:
Ag2CrO4 ⇌ 2Ag+ + CrO42–
1 2 1
x mol/dm3 2x mol/dm3 x mol/dm3
2×10−4 mol/dm3 2 (2×10−4)mol/dm3 2×10−4
On the basis of this equation, the concentration of Ag+ ion will be double of the concentration CrO42– ions.
[Ag+] = 2 × 2 ×10−4
M = 4 ×10−4 M
[CrO42−] = 2 ×10−4 M
Ksp = [Ag+]2[CrO42−] =
[4 ×10−4 M]2 [2 ×10−4 M] = 3.2×10−11 M3
Calculating precipitation occurs or not from Text Book
Calculating precipitation occurs or not from Past Papers
