Fractional Precipitation Pogil Answer Key Best · Recent & Extended

A solution contains 0.010 M Ba²⁺ and 0.010 M Ca²⁺. A student slowly adds solid Na₂CO₃ (which dissociates into 2Na⁺ + CO₃²⁻).

a) Which ion precipitates first?
b) What is the [CO₃²⁻] required to begin precipitation of the first ion?
c) When the second ion just begins to precipitate, what fraction of the first ion remains in solution?

Removing heavy metals (Pb²⁺, Cd²⁺) from wastewater by adding sulfide (S²⁻) relies on huge Ksp differences.

POGIL activities often include metacognitive questions. Here’s how a high-quality answer key addresses frequent errors.

Question: A common mistake is to assume the ion with the smaller (K_sp) always precipitates first regardless of concentration. Is that true? Explain.

Model Answer:
No. The order of precipitation depends on both (K_sp) and initial concentrations. For two salts with the same stoichiometry (e.g., both 1:1), compare the required [Ag⁺] as we did above. If the (K_sp) values are very close, or if the smaller-(K_sp) salt has an extremely low initial concentration, the order could reverse. Always calculate the threshold concentration of the precipitating ion. fractional precipitation pogil answer key best

Example of reversal:
Suppose [I⁻] = (1.0 \times 10^-10 M) and [Cl⁻] = 0.10 M. Then:

Fractional precipitation is often visualized on a log-concentration diagram. The best answer keys include annotated graphs showing:

Plot of [ion] remaining vs. volume of precipitating agent → two distinct drop regions. First drop = Ag⁺ removal, second drop = Pb²⁺ removal.

Calculate [Cl⁻] needed to start precipitation of each:

For AgCl:
[ K_sp = [\textAg^+][\textCl^-] \implies [\textCl^-] = \fracK_sp[\textAg^+] = \frac1.8\times10^-100.01 = 1.8\times10^-8\ \textM ] A solution contains 0

For PbCl₂:
[ K_sp = [\textPb^2+][\textCl^-]^2 \implies [\textCl^-] = \sqrt\fracK_sp[\textPb^2+] = \sqrt\frac1.7\times10^-50.01 = \sqrt1.7\times10^-3 \approx 0.041\ \textM ]

Conclusion: AgCl requires much less Cl⁻ → AgCl precipitates first.

Typical POGIL Question:
You have a solution containing Ba²⁺ (0.10 M) and Sr²⁺ (0.10 M). Which anion—SO₄²⁻ or C₂O₄²⁻ (oxalate)—would allow fractional precipitation?
(K_sp) BaSO₄ = (1.1 \times 10^-10), SrSO₄ = (3.2 \times 10^-7)
(K_sp) BaC₂O₄ = (1.6 \times 10^-6), SrC₂O₄ = (5.6 \times 10^-8)

Model Answer:

First, calculate for sulfate:

BaSO₄ precipitates first (lower required [SO₄²⁻]). The ratio of required concentrations is ~2900:1 — excellent separation.

Now for oxalate:

Here, SrC₂O₄ precipitates first (smaller required [C₂O₄²⁻]). But the required concentrations are very close (ratio only ~28:1). Complete separation would be difficult.

Best choice: Use sulfate. The larger difference in (K_sp) values favors better fractional precipitation.