Chem 212 Finding Qsp and Ksp

Question

Does any solid Ag₂CrO₄ form when 7.0  10^-5 g of AgNO₃ is dissolved in 15 mL of 4.7  10^-3 M K₂CrO₄? Assume no volume change.

Yes, precipitate will form.

No, precipitate will not form.     

Find,

Q_sp

_??

 

K_sp

??

Answer 1

To determine if a precipitate forms, we compare Q to Ksp. If Q is greater than Ksp, then a precipitate will form. Otherwise, no.

Ag₂CrO₄(s) <==> 2Ag⁺(aq) + CrO₄^2-(aq)

Ksp = [Ag⁺]²[CrO₄^2-]

Now we need to find the [Ag⁺]² and [CrO₄^2-]

[Ag⁺] = 7.0x10^-5 g AgNO3 x 1 mol AgNO3 / 170 g = 4.1x10-7 mols / 0.015 L = 2.7x10^-5 M

[CrO₄^2-] = 4.7x10^-3 M

Q = (2.7x10^-5 )²(4.7x10^-3 ) = 3.5x10^-12

Ksp = 1.1x10^-12 (I looked it up. Your value may be different)

Will a precipitate form? YES because Q > Ksp, i.e. 3.5x10^-12 > 1.1x10^-12