### Equilibrium - Watchung Hills Regional High School

```Equilibrium
1994A
Teddy Ku
1994 A
•
•
MgF2(s)
Mg2+(aq) + 2F-(aq)
In a saturated solution of MgF2 at 18 degrees
Celsius, the concentration of Mg2+ is 1.21x103
molar. The equilibrium is represented by the
equation above.
a) Write the expression for the solubility-product
constant, Ksp, and calculate its value at 18
degrees Celsius.
 Ksp = [Mg2+] [F-]2
 [F-] = 2(1.21x10-3)
 Ksp = (1.21x10-3)(2.42x10-3)2 = 7.09x10-9
1994 A
•
•
b)
MgF2(s)
Mg2+(aq) + 2F-(aq)
In a saturated solution of MgF2 at 18 degrees Celsius, the concentration
of Mg2+ is 1.21x103 molar. The equilibrium is represented by the equation
above.
Calculate the equilibrium concentration of Mg2+ in 1.000 liter of saturated
MgF2 solution at 18 degrees Celsius to which .100 mole of solid KF has
been added. The KF dissolves completely. Assume the volume change is
negligible.
MgF2(s)
Mg2+(aq) + 2F-(aq)
Initial (M)
0
Change (M)
0
-x
Equilibrium (M)
0
1.21x10-3 - x
1.21x10-3
2.42x10-3 + .100
-2x
2.42x10-3 + .100 - 2x
Kc = [Mg2+] [F-]2 = (1.21x10-3 - x)(2.42x10-3+.100 - 2x)2 = 7.09x10-9
Solve for x on calculator
X= 1.20929x10-3
[Mg2+] = 1.21x10-3 + x = 2.42x10-3
•
•
c)
•
•
•
1994 A
MgF2(s)
Mg2+ + 2F-(aq)
In a saturated solution of MgF2 at 18 degrees Celsius, the
concentration of Mg2+ is 1.21x103 molar. The equilibrium is
represented by the equation above.
Predict whether a precipitate of MgF2 will form when 100.0
milliliters of a 3.00x10-3 molar Mg(NO3)2 solution is mixed with
200.0 milliliters of a 2.00x10-3 molar NaF solution at 18 degrees
Celsius. Calculations to support your prediction must be shown.
Set up the equilibrium law and calculate (did in a)
–
Ksp=7.09x109
Calculate ion product
–
(3.00x10-3)(2.00x10-3)2=1.2x10-8
Compare ion product with Ksp
–
If the ion product > Ksp, the solution is supersaturated, and
precipitate will form.
–
If the ion product is < or = to the Ksp, no precipitate will form.
–
1.2x10-8 < 7.09x109
•
No precipitate
1994 A
•
•
MgF2(s)
Mg2+ + 2F-(aq)
In a saturated solution of MgF2 at 18 degrees Celsius, the concentration
of Mg2+ is 1.21x103 molar. The equilibrium is represented by the
equation above.
At 27 degrees the concentration of Mg2+ in a saturated solution of MgF2 is
1.17x10-3 molar. Is the dissolving of MgF2 in water an endothermic or an
exothermic process? Give an explanation to support your conclusion
Equilibrium shifts to the left when temperature increases
d)
•
–
•
Heat must be a product
Exothermic if heat is a product
```