The #K_sp# of #Al(OH)_3# is #2*10^-32#. At what pH will a 0.9 M #Al^(3+)# solution begin to show precipitation of #Al(OH)_3#?

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Answer 1 · 2017-02-18T06:58:49

#pH = 2.9718077193 or 3#(rounded off)

Solution

Write the disassociation equation

#Al(OH)_3 ⇌ Al^(3+) + 3OH^-#

The Ksp expression

#Ksp = [Al3^+] [3OH¯]^3#

Plug in the Ksp expression

#2.0 * 10^(-32) = (0.9)(3x)^3#

Solve for# x# which is the concentration of# OH^-#

#0.00000000000000000000000000000002 = 0.9 (3x)^3#

x =# 9.37147E - 12#

Now calculate #"pOH"# from #OH^- #

#pOH = -log(9.37147E -12M)#

#pOH = 11.0281922807#

#pH = 14 - 11.0281922807#
#pH = 2.9718077193 or 3#(rounded off)