Fractional precipitation is a lab technique used to separate multiple ions in a solution by adding a reagent that causes one ion to precipitate before the others. Chemistry Coach Order of Precipitation : The ion that forms the compound with the cap K sub s p end-sub
Understanding Fractional Precipitation: A Guide to the POGIL Activity Answer Key
[Ag+]=8.5×10-170.10=8.5×10-16 Mopen bracket cap A g raised to the positive power close bracket equals the fraction with numerator 8.5 cross 10 to the negative 17 power and denominator 0.10 end-fraction equals 8.5 cross 10 to the negative 16 power M For Silver Chloride ( AgClcap A g cap C l
To solve this section of the answer key, set the reaction quotient ( ) equal to Kspcap K sub s p end-sub fractional precipitation pogil answer key
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.
Do not forget to square or cube concentrations in the Kspcap K sub s p end-sub
[Ag+]required for Ag2CrO4=Ksp(Ag2CrO4)[CrO42−]open bracket Ag raised to the positive power close bracket sub required for Ag sub 2 CrO sub 4 end-sub equals the square root of the fraction with numerator cap K sub s p end-sub open paren Ag sub 2 CrO sub 4 close paren and denominator open bracket CrO sub 4 raised to the 2 minus power close bracket end-fraction end-root 3. Determine Which Substance Precipitates First Compare the two calculated Fractional precipitation is a lab technique used to
Let’s assume a standard POGIL scenario: You have a solution containing . You slowly add 0.1 M HCl (source of Cl⁻ ions). Relevant Ksp values:
When the solutions mix, two potential precipitates can form via double replacement reactions:
The fractional precipitation POGIL activity takes students on a guided inquiry journey from basic Kₛₚ principles to complex multi-ion separation problems. This answer key serves as a framework for instructors and a verification tool for students, but the true value of POGIL lies in the collaborative process of discovery itself. When students successfully work through these calculations and reasoning exercises with their peers, they develop not just knowledge of fractional precipitation, but critical thinking and problem-solving skills that extend far beyond the chemistry classroom. The separation techniques mastered here find applications in fields as diverse as pharmaceutical manufacturing, environmental remediation, and materials science, making fractional precipitation a truly essential concept in the chemist's toolkit. A: All Ksp values are extremely small, but
"The range of ([Cl^-]) for successful separation is from (1.8\times10^-8 M) (start AgCl) to (0.041 M) (start PbCl_2)."
cap C u open paren cap N cap O sub 3 close paren sub 2 open paren a q close paren plus cap N a sub 2 cap C cap O sub 3 open paren a q close paren right arrow cap C u cap C cap O sub 3 open paren s close paren plus 2 cap N a cap N cap O sub 3 open paren a q close paren 3. Determine Which Ion Precipitates First The ion that forms the less soluble salt (the one with the cap K sub s p end-sub ) will precipitate first. Condition for Precipitation: A precipitate begins to form when the reaction quotient ( ) exceeds the solubility product ( cap K sub s p end-sub For example, if cap K sub s p end-sub cap Z n cap C cap O sub 3 , precipitation starts once exceeds this value. Course Hero 4. Use Reaction Quotients ( cap K sub s p end-sub No precipitate forms; the solution is undersaturated. A precipitate forms until equilibrium is reached. Khan Academy 5. Calculate Remaining Ion Concentration