Question

The hydroxide ion has the formula OH−. The solubility-product constants for three generic hydroxides are given here.

Generic hydroxide	Ksp
XOH	3.10×10−8
Y(OH)2	1.20×10−10
Z(OH)3	8.40×10−15

The removal of an ion is sometimes considered to be complete when its concentration drops to 1.00×10−6 M. What concentration of hydroxide would cause Y2+ to "completely" precipitate from a solution? find [OH−] =

2.How many grams of dry NH4Cl need to be added to 2.20 L of a 0.400 M solution of ammonia, NH3, to prepare a buffer solution that has a pH of 8.65? Kb for ammonia is 1.8×10−5. find mass of NH4Cl =

Answer 1

1 )

Y(OH)2   --------------------> Y+2 + 2OH-

                                          S           2 S

Ksp = [Y+2] [OH-]^2

1.20 x 10^-10 = 1.0 x 10^-6 x [OH-]^2

[OH-] = 0.0110 M

2)

moles of NH3 = 2.20 x 0.400 = 0.88

pOH = 14 - pH

        = 14 - 8.65

        = 5.35

pKb = 4.74

pOH = pKb + log [NH4Cl / NH3]

5.35 = 4.74 + log (NH4Cl / 0.88)

NH4Cl / 0.88 = 4.07

NH4Cl moles = 3.58

mass of NH4Cl = 3.58 x 53.5

                         = 192 g