5 Steps to a 5 AP Chemistry

b. 1.00 M Na 2 CrO 4 →1.00 M CrO 42 −(common ion)

Ksp=(2x)^2 (1.00 + x) =1.9 × 10 −^12 = 4 x^2 (neglect x)

x=6.9 × 10 −^6 M

[Ag+] = 2 x=1.4 × 10 −^6 M

Knowing the value of the solubility product constant can also allow us to predict whether a

precipitate will form if two solutions, each containing an ion component of a slightly

soluble salt, are mixed. The ion-product, sometimes represented as Q(same form as the

solubility product constant), is calculated taking into consideration the mixing of the volumes

of the two solutions, and this ion-product is compared to Ksp. If it is greater than Ksp, precip-

itation will occur until the ion concentrations have been reduced to the solubility level.

If 10.0 mL of a 0.100 M BaCl 2 solution is added to 40.0 mL of a 0.0250 M Na 2 SO 4

solution, will BaSO 4 precipitate? Kspfor BaSO 4 =1.1 × 10 −^10

To answer this question, the concentrations of the barium ion and the sulfate ion before

precipitation must be used. These may be determined simply from Mdil=MconVcon/Vdil.

For Ba^2 +: Mdil=(0.100 M)(10.0 mL)/(10.0 +40.0 mL) =0.0200 M

For SO 42 −: Mdil=(0.0250 M)(40.0 mL)/(50.0 mL) =0.0200 M

Entering these values into the following relation produces:

Q =[Ba^2 +][SO 42 −] =(0.0200)(0.0200) =0.000400

Since Qis greater than Ksp, precipitation will occur.

Other Equilibria

Other types of equilibria can be treated in much the same way as the ones discussed above.

For example, there is an equilibrium constant associated with the formation of complex

ions. This equilibrium constant is called the formation constant, Kf. Zn(H 2 O) 42 +reacts

with ammonia to form the Zn(NH 3 ) 42 +complex ion according to the following equation:

The Kfof Zn(NH 3 ) 42 +(aq) is 7.8 × 108 , indicating that the equilibrium lies to the right.

Experimental

Equilibrium experiments such as 10, 11, and 19 in Chapter 19, directly or indirectly involve

filling a table like the following:

Reactants and Products

Initial amount

Change

Equilibrium amount

The initial amounts—concentrations or pressures—are normally zero for the products, and

a measured or calculated value for the reactants. Once equilibrium has been established, the

Zn(H O) aq 242 ++()++ 44 NH aq) 3 (Zn(NH 3 )^24 H O(l) 2

Ag CrO s 24 2 Ag ag CrO 4 2–ag

2100

() () ()

.

 + +

xx+

230  Step 4. Review the Knowledge You Need to Score High

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