), ensure you square the ion concentration in your equilibrium expression. Remember that if , no precipitate forms. If , the solution is exactly saturated. If
| 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 (K sp) vs. (Cu(OH) 2). | | (Cl^-) & (Br^-) | (AgNO_3) | (AgBr) | (AgBr) has lower (K sp) than (AgCl). |
: The solution is at equilibrium (saturated). Precipitate is just about to form. If : The solution is supersaturated. A precipitate will form. 3. Step-by-Step Separation Mechanics
value: The compound is less soluble and precipitates easily. High Kspcap K sub s p end-sub fractional precipitation pogil answer key
The [Cl⁻] added is roughly (10 mL × 0.1 M) / 110 mL total ≈ 0.009 M. This exceeds the threshold for Hg₂²⁺ and Ag⁺ but is much lower than the 0.0412 M needed for Pb²⁺.
will precipitate first because it is much less soluble and requires a far lower concentration of Ag+cap A g raised to the positive power ions to exceed its Kspcap K sub s p end-sub threshold. Problem 2: Calculate the required to begin precipitation for each ion.
Students are presented with a beaker containing two different anions (e.g., Cl−cap C l raised to the negative power I−cap I raised to the negative power ), ensure you square the ion concentration in
4. Calculate Remaining Concentration at the Second Precipitation Point
to find the exact point where the solution becomes saturated. For Silver Iodide ( AgIcap A g cap I
A typical POGIL on fractional precipitation presents you with: If | Ion Pair | Possible Precipitant |
AmBn(s)⇌mAn+(aq)+nBm−(aq)cap A sub m cap B sub n open paren s close paren is in equilibrium with m cap A raised to the n plus power open paren a q close paren plus n cap B raised to the m minus power open paren a q close paren Kspcap K sub s p end-sub expression is written as:
The ion with the smallest Ksp (solubility product constant) will precipitate at the lowest concentration of the precipitating agent.
represents the equilibrium constant for a solid substance dissolving in an aqueous solution. It is a measure of the extent to which a compound can dissolve.For a general dissolution equation:
[Ag+]=1.8×10-9 Mopen bracket cap A g raised to the positive power close bracket equals 1.8 cross 10 to the negative 9 power M Because AgBrcap A g cap B r requires a much lower concentration of Ag+cap A g raised to the positive power to begin precipitating ( ), AgBrcap A g cap B r will precipitate first.