Question

need help with these two questions

QUESTIONS 1. Based on the value of the Kap that you calculated for AsyCr04. how many grams of Axoro. dissolve per Liter of solution? 2. In the second part of the experiment, why was the AgNO3] larger than the IK,Cro,l? What would have happened if they had been the same?

Answer 1

Multiple question as per guideline only first should be answered, but for convenience of student both questions are answered :

1. We have, Ksp (solubility product) = [Ag⁺]² [CrO₄^2-]

We will take your calculated value as per question :

Ksp (solubility product) = [Ag⁺]² [CrO₄^2-] = 1.77*10^-14

How mang grams of Ag₂CrO₄ will dissolve per litre of solution ?

or we have to calculate molar solubility :

Ag₂CrO₄  $\rightleftharpoons$ 2 Ag⁺ + CrO₄^2-

Initial concentration (M) 1    0 0

Change (M) Negligible +2s + s ( s = solubilty of Ag₂CrO₄)

At Equilibrium   ~1 2s s   

At Equilibrium    Ksp = [Ag⁺]² [CrO₄^2-]   

or     Ksp = [Ag⁺]² [CrO₄^2-] = (2s)² (s) = 4s³

or    1.77*10^-14 = 4s³

Solubility (s) = 1.64*10^-5 M

Molar mass of Ag₂CrO₄ = 331.73 g/mol

So, amount of Ag₂CrO₄ dissolved = 1.64*10^-5 mol/L *331.73 g/mol = 5.45*10^-3 g/L

2. When we mix solutions of AgNO₃ and K₂CrO₄ following reaction takes place :

2 AgNO₃ (aq) +  K₂CrO₄ (aq) $\rightarrow$   Ag₂CrO₄(s) + 2 K⁺(aq) + 2 NO₃^- (aq)

Net ionic reaction :  2 Ag⁺ (aq) + CrO_4^2-(aq) $\rightarrow$   Ag₂CrO₄(s)

So, it is evident that 2 moles of 2 Ag⁺ reacts with one mole of  CrO_4^2-(aq), so we had [AgNO₃] larger than .[K₂CrO₄].

As per Ksp we have ions in solution :

[Ag⁺] = 3.28* 10^-5   and [CrO₄^2-] = 1.64*10^-5

After formation of  Ag₂CrO₄ we have following equilibrium ;

Ag₂CrO₄(s) $\rightleftharpoons$  2 Ag⁺ (aq) + CrO_4^2-(aq) ;   Ksp = [Ag⁺]² [CrO₄^2-]

If, we add equal amount of both than Ag⁺ will act as limiting reactant and almost all of Ag⁺ will be in form of  Ag₂CrO₄(s) and Ksp ( product of of ionic concentrations in solution)   will product of higher concentration of [CrO₄^2-] and very lower concentration of  [Ag⁺] ( product of of ionic concentrations in solution) ; as Ksp is a constant value irrespective of amount of  Ag₂CrO₄.  Ag₂CrO₄(s) starts forming only after reaction Quotient exceeds solubility product.