Lab · Qualitative Analysis of Ions · AP Chemistry

A three-cation crime scene. A single test tube holds an unknown mix of Fe³⁺, Ba²⁺, and Ag⁺and your job is to prove which are present. You'll use solubility rules to knock one ion out of solution at a time (selective precipitation), then run a confirmatory test on each suspect to make the ID stick. This is CSI chemistry: separate, then confirm.

Time: 1 block (~75 min) · Scale: semimicro (drops) · Type: formative, graded on reasoning & flow-chart, not on a "right" unknown

Purpose & AP Connection CED 3.0 · SP 6.4

Purpose: develop a separation scheme and confirmatory tests for Fe³⁺, Ba²⁺, and Ag⁺, then use that scheme to identify which of these cations are present in an unknown solution.

CED tie-in: Unit 3 (Intermolecular Forces & Properties), the solubility of ionic salts in water, and how ion–water and ion–ion interactions decide what stays dissolved. The reasoning also reaches into solubility equilibria (K_sp, Unit 7). The College Board "recommended experiment" this maps to is #14, Separation and qualitative analysis of cations and anions.

Science Practice 6.4: Make a scientific claim and support it with evidence. Every ion you report must be backed by a specific observation (a colored precipitate, a color change) and the rule that explains it. "It turned red, therefore Fe³⁺" is the whole game.

Background solubility · selective precipitation

Qualitative analysis asks which ions are present, not how much. With a mixture of cations, you can't just test the whole tube at once, adding a confirmatory reagent could give two reactions and a muddy answer. So you separate first.

Selective precipitation: add an anion that pairs with only one cation to form an insoluble solid. That ion drops out as a precipitate while the others stay dissolved. Pour off the liquid (the supernatant) and you've physically split the mixture.

The brief solubility rules we use (Reger, Goode & Mercer): 1. all nitrates soluble · 2. all chlorides soluble except AgCl, Hg₂Cl₂, PbCl₂ · 3. all sulfates soluble except SrSO₄, BaSO₄, CaSO₄, PbSO₄ · 4. carbonates insoluble except group IA & NH₄⁺ · 5. hydroxides insoluble except group IA, NH₄⁺, Sr²⁺, Ba²⁺.

The plan (each cation leaves at its own step): Step 1, HCl: Cl⁻ precipitates AgCl (white) by rule 2; Fe³⁺ & Ba²⁺ stay dissolved. Step 2, NH₃: makes the solution basic; OH⁻ precipitates Fe(OH)₃ (rusty/red-brown) by rule 5; Ba²⁺ stays (rule 5 exception). Step 3, H₂SO₄: SO₄²⁻ precipitates BaSO₄ (white) by rule 3.

Confirmatory tests (each ion has a fingerprint): Ag⁺ → re-precipitates white AgCl when Cl⁻ is present (and darkens in light). Fe³⁺ → forms a blood-red complex with thiocyanate, [Fe(SCN)]²⁺, with NaSCN. Ba²⁺ → forms a fine white BaSO₄ precipitate with H₂SO₄.

Safety read before you draw a single drop

Corrosives: 6 M HCl, 6 M H₂SO₄, 6 M HNO₃ are strong acids; 7.5 M NH₃ is a strong base. All attack skin and can cause permanent eye damage. Splash to eyes → eyewash immediately, hold lids open, flush 15 min, tell Dr. Mulinta. Splash to skin/clothing → flush with water.

Vapors: these reagents have irritating fumes. Dispense NH₃, HCl, and HNO₃ at the fume hood when possible; do not inhale; never wave a hand over an open bottle toward your face.

Silver nitrate: AgNO₃ stains skin dark (spots appear ~24 h later, harmless, fade in days) and ruins clothing. Wash any contact off promptly.

PPE, required the entire period: splash goggles (not glasses), apron, nitrile gloves (set out for the acid/base work), closed-toe shoes, tied-back hair. No food or drink.

Waste: everything contains heavy-metal cations, nothing goes down the sink. Pour all liquids, rinses, and solids into your bench waste beaker, then into the central waste container in the hood.

Materials & Equipment low-gear · semimicro

Per pair. We run drop-scale, so glassware is small and quantities are tiny. Lab tech Kathy will pre-set the dropper-bottle reagent station and the labelled waste beakers before class.

Equipment (per pair): • 6 small test tubes (10×75 mm) in a rack • 1 plastic well plate (the 4×6 spot plate) • 3 glass stirring rods • 1 wash/squirt bottle of deionized water • 1 labelled 250 mL "WASTE" beaker • test-tube brush; wax pencil/labels; paper towel • Kathy pre-sets: the centrifuge station if availableif no centrifuge, we let precipitates settle by gravity (5–10 min) and decant carefully

Reagents, in labelled dropper bottles (Kathy pre-fills): • 0.10 M Fe(NO₃)₃ • 0.10 M Ba(NO₃)₂ • 0.10 M AgNO₃ • the unknown solutions (numbered) • 6 M HCl • 7.5 M NH₃ • 6 M H₂SO₄ • 6 M HNO₃ • 0.05 M NaSCN • deionized water

Everyday-gear note: a tabletop centrifuge speeds settling but is optional, gravity settling works on these dense precipitates. AgCl and BaSO₄ are heavy and drop within minutes; Fe(OH)₃ is gelatinous and slower, so give it the full settle time before decanting.

Procedure drops, not mL · record as you go

Technique reminders: never let a dropper tip touch your solution or the tube wall (contamination = false positive). "Add until no more solid forms" means: add a drop, watch, repeat until one more drop makes no new cloudiness. To decant, let the solid settle (or centrifuge), then pour the clear liquid into a clean tube, leaving the solid behind. Wash a precipitate by adding ~0.5 mL DI water, stirring to break it up, re-settling, and decanting that rinse to waste.

Part A: Confirmatory tests on the known ions

On the well plate, set up a 3×3 grid. Columns = the three cations: column 1 = 3 drops Fe³⁺ in each of 3 wells; column 2 = 3 drops Ba²⁺ in each; column 3 = 3 drops Ag⁺ in each.
Row 1: add 2 drops 6 M HCl to each cation. Record what you see (precipitate? color?) in Data Table A.
Row 2: add 1 drop 0.05 M NaSCN to each cation. Record.
Row 3: add 2 drops 6 M H₂SO₄ to each cation. Record.
Keep this well plate, it's your reference card for matching the unknown later.

Part B: Separation scheme on a known mixture (all three ions)

In a clean test tube, combine 3 drops each of Fe³⁺, Ba²⁺, and Ag⁺.
Add 6 M HCl drop by drop until no more precipitate forms (test with one extra drop). Let settle / centrifuge. This is ppt. #1.
Decant the supernatant into a clean tube. Label and set aside the solid as ppt. #1.
To that supernatant, add 7.5 M NH₃ (start with ~10 drops) until no more solid forms; confirm with one extra drop. Settle / centrifuge. This is ppt. #2.
Decant the supernatant into a clean tube. Label and set aside the solid as ppt. #2.
To that supernatant, add 6 M H₂SO₄ drop by drop until no more solid forms; confirm with one extra drop. Settle / centrifuge. This is ppt. #3.
Confirm ppt. #1 (Ag⁺): wash ppt. #1 with ~0.5 mL cold DI water, stir, settle, decant to waste. Add 3 drops 7.5 M NH₃ + ~0.5 mL water, stir to dissolve, settle, and decant the liquid into a clean dry tube (discard the solid). To that liquid add 3 drops 6 M HNO₃, a white precipitate re-forming = AgCl reappearing = Ag⁺ confirmed.
Confirm ppt. #2 (Fe³⁺): add 3–5 drops 6 M HCl to ppt. #2 until it dissolves, then add 1 drop 0.05 M NaSCN. A blood-red color = Fe³⁺ confirmed.
Confirm ppt. #3 (Ba²⁺): the white solid that formed with H₂SO₄ in step 6 is BaSO₄ = Ba²⁺ confirmed.
Complete the Figure 1 flow chart on your data sheet, then send all tubes to waste and rinse.

Part C: Identify the unknown

Record your unknown number on the data sheet. Place 9 drops of the unknown in a clean test tube.
Run the exact same scheme as Part B (steps 2–9), recording every observation in the Figure 2 flow chart. An ion is "present" only where you see the precipitate/color your known scheme predicted, a negative result (no precipitate, no color) means that ion is absent.
Report which of Fe³⁺, Ba²⁺, Ag⁺ are present, each justified by its confirmatory observation.

Data Tables fill in as you work

Data Table A, Confirmatory tests on known ions. Record color and whether a precipitate (ppt.) forms.

Reagent added	Fe³⁺ (yellow soln.)	Ba²⁺ (colorless)	Ag⁺ (colorless)
2 drops 6 M HCl
1 drop 0.05 M NaSCN
2 drops 6 M H₂SO₄

Data Table B, Separation of the known mixture.

Step / reagent	Precipitate? color?	Supernatant color	Ion separated
+ HCl (ppt. #1)
+ NH₃ (ppt. #2)
+ H₂SO₄ (ppt. #3)
ppt. #1 → HNO₃ confirm
ppt. #2 → HCl + NaSCN confirm

Data Table C, Unknown #______. Same scheme; note PRESENT / ABSENT for each branch.

Step / reagent	Observation	Ion	Present / Absent
+ HCl		Ag⁺
+ NH₃		Fe³⁺
+ H₂SO₄		Ba²⁺
confirmatory (NaSCN / HNO₃)

Figure 1 / Figure 2, Flow chart. Sketch the separation tree below: start with the mixture, branch at each reagent into "precipitate" (the ion that left) and "supernatant" (what carries on). Label every solid with its formula, color, and confirmatory test. Draw Figure 1 for the known mixture and Figure 2 for your unknown.

Mixture: Fe³⁺ + Ba²⁺ + Ag⁺

──┬── + HCl → ppt. #1: ____________ (color ____)   confirm: ____________
     supernatant ↓
──┬── + NH₃ → ppt. #2: ____________ (color ____)   confirm: ____________
     supernatant ↓
───── + H₂SO₄ → ppt. #3: ____________ (color ____)   confirm: ____________

Analysis Questions answer in complete sentences

From Data Table A, which ion(s) precipitate when HCl is added? Which rule predicts it?
From Data Table A, which ion(s) precipitate when H₂SO₄ is added? Which rule predicts it?
In the separation (Part B), after adding HCl: (a) which ion precipitates, and (b) which ion(s) remain in the supernatant?
After adding NH₃ to the supernatant: (a) which remaining ion precipitates in basic solution (cite the rule), and (b) which ion stays dissolved and why (cite the rule's exception)?
Why must you wash ppt. #1 with water before its confirmatory test? What kind of error does skipping the wash cause?
Write the net ionic equation for the reaction when HCl is added to the mixture (the AgCl precipitation).
Write the net ionic equation for the reaction when the solution is made basic with NH₃ and Fe(OH)₃ precipitates.
Write the net ionic equation for the reaction when H₂SO₄ precipitates BaSO₄.
K_sp reasoning: AgCl has K_sp = 1.8×10⁻¹⁰. For a saturated solution of AgCl in pure water, calculate the molar solubility (mol/L of Ag⁺). Show your work.
Stoichiometry of a "complete" precipitation: Suppose your unknown tube held 9 drops at 0.10 M Ag⁺, and 1 drop ≈ 0.050 mL. How many moles of Ag⁺ are present, and what minimum moles of Cl⁻ are needed to precipitate it all? Show the calculation.
Describe the location of each cation in your completed flow chart and the confirmatory test you used to prove it.
A classmate reports "Ba²⁺ present" because their tube went cloudy white after HCl in step 2. Explain the likely error and what white precipitate they actually saw.
Error analysis, false positive vs. false negative (the Ag⁺ branch). Describe one technique error that could make you wrongly report an ion as present (a false positive) and one that could make you miss an ion that truly is present (a false negative). Explain each through the relevant precipitation/confirmation equilibrium (e.g. AgCl(s) ⇌ Ag⁺ + Cl⁻).
Error analysis, contamination vs. loss. Give one error that produces a false positive on the Fe³⁺ or Ba²⁺ confirmation and one that produces a false negative for an ion that is present. Tie each to the precipitation or complex-formation equilibrium involved.