2.3.2 Will A Precipitate Form?
Find the concentration of an ion found in two equal volumes that are mixed
Example 3
Equal volumes of 0.080 M BaCl2 and 0.020 M Na2SO4 are mixed. Find the final concentrations of each of the ions present.
Solution
Mixing equal volumes (initially) halves every concentration:
Na2SO4(aq) → 2Na+(aq) + SO42–(aq)
Initial concentrations 0.020 M → 0.040 M 0.020 M
Conc. after mixing 0.020 M 0.010 M
BaCl2(aq) → Ba2+(aq) + 2Cl–(aq)
Initial concentrations 0.080 → 0.080 M 0.160 M
Conc. after mixing 0.040 M 0.080 M
The final [Na+(aq)] = 0.02 M, and [Cl–(aq)] = 0.08 M, since these ions will not precipitate.
Now for Ba2+(aq) and SO42–(aq):
BaSO4(s) ⇌ Ba2+(aq) + SO42–(aq)
Conc. just after mixing — 0.040 M 0.010 M
Part that precipitates. x M x M
Conc. left after mixing 0.040 – x 0.010 – x
Because the Ksp = 1.5 × 10–9 is small, x must be practically 0.01 M (almost all the SO42–(aq) reacts), so:
[Ba2+(aq)] = 0.040 – 0.010 = 0.030 M.
Now x can be calculated from the Ksp:
Ksp = [Ba2+(aq)].[SO42–(aq)]
[SO42–(aq)] = Ksp / [Ba2+(aq)] = (1.5 × 10–9) / 0.030 = 5.0 × 10–8 M
This confirms that our assumption about x being very small is correct.
2.3.3 Precipitations Used For Separations
A chemist is often interested in separating substances in a solution mixture. Such a problem is solved by applying equilibrium considerations. Suppose we have a solution known to contain both lead nitrate, Pb(NO3)2, and magnesium nitrate, Mg(NO3)2. The lead and magnesium can be separated by removing from the solution all of the lead ion, Pb2+(aq), as a precipitate. We must avoid, of course, precipitation of any magnesium compound. We know that Pb2+ and SO42– form a compound with low solubility. If enough sodium sulfate, Na2SO4, is added, lead sulfate, PbSO4, will precipitate. Since magnesium sulfate, MgSO4, is soluble, there will be no precipitate of MgSO4. The solid can be removed from the liquid by filtration and the desired separation has been obtained.
Which of the following soluble salts would permit a separation of magnesium and lead through a precipitation reaction? Choose the correct answer(s).
sodium iodide , NaI
sodium carbonate , Na2CO3
sodium sulfide , Na2S
Which of the following soluble salts would permit a separation of magnesium and lead through a precipitation reaction? Choose the correct answer(s).
sodium iodide , NaI
sodium carbonate , Na2CO3
