Question

5. Which of the following when dissolved in water has the highest concentration of calcium ions?

A. CaCO_3;          K_sp = 8.7 x 10^-9

B. CaF₂;          K_sp = 4.0 x 10^-11

C. Ca(OH)₂     K_sp = 8.0 x 10^-6

D. Ca₃(PO₄)₂   K_sp = 4.0 x 10 ^-11

36. For which type of titration will the pH be acidic at the equivalence point?

Strong acid vs. strong base.

Strong acid vs. weak base.

Weak acid vs. strong base.

All of the above.

None of the above.

Answer 1

5)
for CaCO3:
CaCO3 <----> Ca2+ + CO32-
                              s               s
Ksp = s*s
8.7*10^-9 = s^2
s = 9.33*10^-5 M
[Ca2+] = 9.33*10^-5 M

------------------------
for CaF2:
CaF2 <----> Ca2+ + 2F-
                           s            2s
Ksp = s*(2s)^2
4.0*10^-11 = 4*s^3
s = 2.15*10^-4 M
[Ca2+] = 2.15*10^-4 M
-------------------------------
for Ca(OH)2 again,
Ksp = 4*s^3
8.0*10^-6 = 4*s^3
s =0.0126 M
[Ca2+] = 0.0126 M
----------------
for Ca3(PO4)2 :
Ca3(PO4)2   <----> 3Ca2+ + 2 PO43-
                                     3s                  2s

Ksp = (3s)^3 (2s)^2
4.0*10^-11 = 108*s^5
s = 3.26*10^-3 M
[Ca2+] = 3s = 3*3.26*10^-3 =0.0098 M

Answer: b) Ca(OH)2

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