Question

1) 20.0 mL of 0.01 M AgNO3 and 30.0 mL of 0.010 M K2CrO4 are mixed together and allowed to react. Will a precipitate form? What will be the molarities of the silver ion and chromate ion at equilibrium?

2) Tooth enamel is composed of hydroxyapatite, whose simplest formula is Ca5(PO4)3OH and whose corresponding Ksp = 6.8 x 10^-27. As discussed, fluoride in fluorinated water or in toothpaste reacts with the hydroxyapatite to form fluoroapatite, Ca5(PO4)3F whose Ksp = 1.0 x 10^-60.

a) write the expression for the solubility constant for hydroxyapatite and for fluoroapatite.

b) calculate the molarity of each of these compounds.

Answer 1

1) Ag2CrO4(s) <------->2 Ag+ (aq) + CrO42-(aq)

Ksp = [ Ag+ ]^2 [ CrO42-] = 1.12×10^-12

If Qsp > Ksp , precipitate of Ag2CrO4 will form

where , Qsp is Solubility product Quotient ( Current state of ionic concentration of solution )

After mixing total volume = 50ml

After mixing

AgNO3 solution is diluted 2.5 time

[ Ag+ ] = 0.01M/2.5 = 0.004M

K2CrO4 solution is diluted 1.666time

[ CrO42-] = 0.01M / 1.666 = 0.006M

Qsp = [ Ag+ ]^2 [ CrO42- ]

= (0.004)^2 × 0.006

= 9.6 × 10^-8

Qsp > Ksp

So, Precipitation of Ag2CrO4 will form

2) a) Ca5(PO4)3OH(s) <--------> [Ca5(PO4)3]+(aq) + OH-(aq)

Ksp = [ [Ca5(PO4)3]+ ] [ OH-]= 6.28×10^-27

Ca5(PO4)3F(s) <--------> [Ca5(PO4)3]+(aq) + F-(aq)

Ksp = [[ Ca5(PO4)3]+] [ F- ]= 1.0 × 10^-60

b) at saturated solution of Ca5(PO4)3OH

Solubility of Ca5(PO4)3OH = √6.28×10^-27

= 7.92×10^-14mol/L

At saturated solution of Ca5(PO4)3F

Solubility of Ca5(PO4)3F = √1.0×10^-60

= 1.0 ×10^-30 mol/L