Question

A) Without doing any calculations (just compare the correct K_sp values) complete the following statements:

1.	calcium sulfide is	MORE soluble than	ABCD?
2.	calcium sulfide is	LESS soluble than	ABCD?
		A. MgCO3
		B. CaCO3
		C. Fe(OH)2
		D. Pb(OH)2

B) For each of the salts on the left, match the salts on the right that can be compared directly, using K_sp values, to estimate solubilities. If more than one answer is correct, enter the letters without delimiting characters.

	1.	calcium sulfite		A. Ag2CO3
	2.	nickel(II) hydroxide		B. Ba3(PO4)2
				C. AgCN
				D. PbS

Write the expression for K_sp in terms of the solubility, s, for each salt, when dissolved in water.

calcium sulfite			nickel(II) hydroxide
Ksp =			Ksp =

Note: Multiply out any number and put it first in the K_sp expression. Combine all exponents for s.

C) Determine the relative solubilities of the following magnesium compounds:

Compound	Ksp	Relative Solubility, M
MgCO3	4.0×10-5	lowest / intermediate / highest
MgF2	6.4×10-9	lowest / intermediate / highest
Mg(OH)2	1.5×10-11	lowest / intermediate / highest

Answer 1

A)

Solubility increases as Ksp value increases.

The Ksp vaues are as follows,

Compound	Ksp
MgCO3	~3.5 x 10-7.8
CaCO3	~2.8 x 10-9
Fe(OH)2	~8.0 x 10-16
Pb(OH)2	~1.42 x 10-20
CaS	~8 x 10-6

So,

1. Calcium sulfide is MORE soluble than ABCD (Since the Ksp value for CaS is greater than others and we already know, greater the Ksp value, more will be the solubility of that compound)

2. Calcium sulfide is LESS soluble than None

B)

We can compare the Ksp value directly to determine the solubilty of compounds only when the compounds which is compared should have same number of cations and anions.

Here, Ksp(CaSO3) = [Ca2+] [SO32-] = S x S

Ksp(Ni(OH)2) = [Ni2+] [-OH]2 = S x 2S2 (Since it produce 2 -OH on dissociation)

Ksp(Ag2CO3) = [Ag+]2 [CO32-] = 2S2 x S (Since it produce 2 Ag+ on dissociation)

Ksp(Ba3(PO4)2) = [Ba2+]3 [PO42-]2 = 3S3 x 2S2 (Since it produce 3 Ba2+ and 2 PO42- on dissociation)

Ksp(AgCN) = [Ag+] [CN-] = S x S

Ksp(PbS) = [Pb2+] [S2-] = S x S

So here, Ksp of Ni(OH)2 can be directly compared with Ksp of Ag2CO3 to estimate solubility.

And Ksp of CaSO3 can be directly compared with AgCN and PbS to estimate solubility.

Expression for Ksp in terms of the solubility, s, for CaSO3 and Ni(OH)2

Ksp(CaSO3) = [Ca2+] [SO32-] = S x S = S2

Ksp(Ni(OH)2) = [Ni2+] [-OH]2 = S x 2S2 (Since it produce 2 -OH on dissociation) = 4S3

C)

Compound	Ksp	Solubility(in terms of 'S')	Solubility ((relative solubility)
MgCO3	4.0×10-5	S2	6.324 x10-3 (intermediate)
MgF2	6.4×10-9	4S3	7.43 x 10-3 (highest)
Mg(OH)2	1.5×10-11	4S3	9.86 x 10-4 (lowest)