Pogil Answer Key - Fractional Precipitation
Find ([Cl^-]) when ([Ag^+] = 1.0\times 10^-5) M (complete precipitation): [ [Cl^-] = \fracK_sp(AgCl)[Ag^+] \textfinal = \frac1.8\times 10^-101.0\times 10^-5 = 1.8\times 10^-5 \text M ] At this ([Cl^-]), check if (PbCl_2) has started: (Q = [Pb^2+][Cl^-]^2 = (0.10)(1.8\times 10^-5)^2 = 3.24\times 10^-11) Compare to (K sp(PbCl_2) = 1.7\times 10^-5). (Q \ll K_sp), so (Pb^2+) is still in solution. Separation is possible.
is a technique used to separate a mixture of metal ions from a solution. It relies on a key principle: Different ions have different solubilities (Ksp values). By carefully adding a precipitation agent (like chloride, sulfide, or hydroxide ions), you can cause the least soluble compound to precipitate first , leaving the more soluble ions in solution.
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:
is an equilibrium constant that represents the level to which a compound dissolves in water. Represents a less soluble compound (precipitates easily). High Kspcap K sub s p end-sub
[Ag+]required for AgCl=Ksp(AgCl)[Cl−]open bracket Ag raised to the positive power close bracket sub required for AgCl end-sub equals the fraction with numerator cap K sub s p end-sub open paren AgCl close paren and denominator open bracket Cl raised to the negative power close bracket end-fraction fractional precipitation pogil answer key
8.5×10-17=[Ag+](0.10 M)8.5 cross 10 to the negative 17 power equals open bracket Ag raised to the positive power close bracket open paren 0.10 M close paren
To evaluate the efficiency of this fractional precipitation, find out what percentage of the original Br−cap B r raised to the negative power remains in the solution.
starts to precipitate. This indicates an exceptionally successful quantitative separation. Tips for Mastering POGIL Chemistry Activities
Keep practicing, trust the equilibrium, and use guided inquiry to build lasting chemical intuition. Find ([Cl^-]) when ([Ag^+] = 1
(Ion Product): This is calculated using the exact same formula as Kspcap K sub s p end-sub
When a reagent is added to a solution containing multiple ions, it will react with all of them to form potential precipitates. However, the precipitates do not form simultaneously. The compound with the lower solubility will reach its saturation point first and begin to precipitate out of the solution, while the more soluble compound remains dissolved until a higher concentration of the precipitating ion is added. Key Factors Determining Order of Precipitation Kspcap K sub s p end-sub
The Ultimate Guide to Fractional Precipitation: Understanding the POGIL Activity and Answer Key
A common question asks for the concentration of the first ion remaining in solution just as the second ion begins to precipitate. is a technique used to separate a mixture
Fractional precipitation, also known as selective precipitation , is a technique that exploits differences in ion solubility to separate them. By adding a reagent that forms an insoluble salt with specific target ions, and carefully controlling the concentration of that precipitant, it’s possible to remove the ions from the solution in a step-by-step, or fractional, manner.
, but it uses the current concentrations of ions in the solution, rather than the equilibrium concentrations. Predicting Precipitation By comparing Kspcap K sub s p end-sub , you can predict the state of the solution: : The solution is unsaturated. No precipitate forms.
By focusing on the step-by-step logic—calculating the precipitation threshold for each ion and determining the concentration remaining—the concepts in the fractional precipitation POGIL module become much clearer.
A standard POGIL problem involves a solution containing two anions, such as Cl−cap C l raised to the negative power Br−cap B r raised to the negative power , to which a cation like Ag+cap A g raised to the positive power
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.
: 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