Fractional Precipitation - Pogil Answer Key

Let’s assume a standard POGIL scenario: You have a solution containing 0.01 M Ag⁺, 0.01 M Pb²⁺, and 0.01 M Hg₂²⁺. You slowly add 0.1 M HCl (source of Cl⁻ ions). Relevant Ksp values:

If your POGIL activity includes mixed-salt types, use this table.

| Ion Pair | Possible Precipitant | First Precipitate | Why? | | :--- | :--- | :--- | :--- | | (Mg^2+) & (Ca^2+) | (Na_2CO_3) | (MgCO_3) (if (K_sp) smaller) | Calculate actual [CO3^2-] needed. | | (Fe^3+) & (Cu^2+) | (OH^-) | (Fe(OH)_3) | (Fe(OH)3) has extremely low (Ksp) vs. (Cu(OH)2). | | (Cl^-) & (Br^-) | (AgNO_3) | (AgBr) | (AgBr) has lower (Ksp) than (AgCl). | fractional precipitation pogil answer key

Worked Example for the table: A solution is 0.01 M (Fe^3+) and 0.01 M (Cu^2+). (K_sp) (Fe(OH)_3 = 4\times10^-38), (Cu(OH)_2 = 2.2\times10^-20).

For students who want to go deeper, here are additional questions (with short answers) similar to those on advanced POGILs. Let’s assume a standard POGIL scenario: You have

Q: What if we used Na₂S instead of HCl? Ksp: Ag₂S = 6×10⁻⁵⁰, PbS = 8×10⁻²⁸, HgS = 4×10⁻⁵³.
A: All Ksp values are extremely small, but HgS (smallest) precipitates first, then Ag₂S, then PbS. However, all will precipitate almost instantly—poor separation.

Q: How does pH affect fractional precipitation of hydroxides?
A: For metal hydroxides M(OH)₂, Ksp = [M²⁺][OH⁻]². Lower pH (more acidic) means fewer OH⁻ ions; you can selectively precipitate Fe³⁺ (Ksp ~ 10⁻³⁹) before Mg²⁺ (Ksp ~ 10⁻¹¹) by carefully adjusting pH. For students who want to go deeper, here

Q: Why is "fractional precipitation" different from "selective precipitation"?
A: They are often used interchangeably, but selective implies perfect separation; fractional acknowledges that separation is gradual and incomplete.