What is the maximum mass (in grams) of each of the following soluble salts that

Question

What is the maximum mass (in grams) of each of the following soluble salts that can be added to 330 mL of 0.050 M BaCl2 without causing a precipitate to form? (Assume that the addition of solid causes no change in volume.)

(a) (NH4)2SO4


(b) Pb(NO3)2


(c) NaF What is the maximum mass (in grams) of each of the following soluble salts that can be added to 330 mL of 0.050 M BaCl2 without causing a precipitate to form? (Assume that the addition of solid causes no change in volume.)

(a) (NH4)2SO4


(b) Pb(NO3)2


(c) NaF

Explanation / Answer

a) The precipitate that forms is BaSO4: BaCl2(aq) + (NH4)2SO4(aq) ==> BaSO4(s) + 2NH4Cl(aq)

Since [Ba2+] = 0.050 M, then

Ksp BaSO4 = [Ba2+][SO4 2-] = 9.1 x 10^-11 (I looked it up)

[SO4 2-] = (9.1 x 10^-11) / [Ba2+] = (9.1 x 10^-11) / 0.050 = 1.8 x 10^-9 M

moles SO4 2- = Molarity SO4 2- x L solution = (1.8 x 10^-9)(0.330) = 5.94 x 10^-10 moles SO4 2-

5.94 x 10^-10 moles SO4 2- x (1 mole (NH4)2SO4 / 1 mole SO4 2-) x (132 g (NH4)2SO4 / 1 mole (NH4)2SO4) = 7.84 x 10^-8 g (NH4)2SO4 needed to just start to precipitate BaSO4.

The others are done the same way except that the precipitate is different (PbCl2 in part (b) --- Ksp = 1.6 x 10^-5; and BaF2 in part (c) --- Ksp = 1.0 x 10^-6).

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