Mixed Ionic Compound Problems- Answer Key and Explanations
What Are Mixed Ionic Compound Problems?
Mixed ionic compound problems test your ability to name and write formulas for compounds containing both simple cations and polyatomic ions. These aren't just the basic NaCl type problems from chapter one. They throw in transition metals, Roman numerals, and polyatomic ions that make students sweat during exams.
If you're staring at a worksheet full of these problems and feeling lost, here's the thing: the rules don't change. You just have more pieces to track. Let's fix that.
The Naming Framework (Quick Refresher)
Before diving into problems, you need this locked down:
- Simple metal + nonmetal = metal name + ide suffix (e.g., potassium chloride)
- Metal with multiple charges = metal name + (Roman numeral) + nonmetal name + ide (e.g., iron(III) oxide)
- Metal + polyatomic ion = metal name + polyatomic ion name (e.g., sodium sulfate)
- Ammonium + anything = ammonium first, then whatever (e.g., ammonium nitrate)
That's it. Four patterns. Everything in mixed problems is some combination of these.
Mixed Ionic Compound Problems: Practice Set
Try these before checking the answers. Give yourself 2 minutes per problem max. If you're stuck, move on and come back.
Problem 1: Name This Compound
Fe₂O₃
Answer: Iron(III) oxide
Explanation: Fe is iron, a transition metal with variable charge. O is oxygen, always -2. You have 2 iron atoms and 3 oxygen atoms, so total negative charge is -6. That means the iron must contribute +6 total, or +3 per iron atom. Iron(III) tells you the charge is +3. Oxide is oxygen with the -ide suffix.
Problem 2: Write the Formula
Copper(II) chloride
Answer: CuCl₂
Explanation: Copper(II) means Cu²⁺. Chloride is Cl⁻. One copper ion cancels one chloride ion charge-wise, but you need two chloride ions to balance copper's +2 charge. So: Cu²⁺ + 2Cl⁻ = CuCl₂. No subscripts shown for the 1, so it's just Cu.
Problem 3: Name This Compound
Ca(NO₃)₂
Answer: Calcium nitrate
Explanation: Calcium is a Group 2 metal, always +2. Nitrate is NO₃⁻. You need two nitrate ions to balance one calcium ion. Since calcium only has one possible charge, no Roman numeral needed. When a polyatomic ion needs a subscript, put it in parentheses first: Ca(NO₃)₂.
Problem 4: Write the Formula
Ammonium carbonate
Answer: (NH₄)₂CO₃
Explanation: Ammonium is NH₄⁺. Carbonate is CO₃²⁻. Two ammonium ions (+1 each) give you +2 total, which balances carbonate's -2. The subscript goes outside the parentheses for ammonium: (NH₄)₂CO₃. Notice carbonate doesn't need parentheses because it's only used once.
Problem 5: Name This Compound
Pb(SO₄)₂
Answer: Lead(IV) sulfate
Explanation: Pb is lead. SO₄ is sulfate, -2 charge. You have two sulfate ions, so total negative charge is -4. Lead must be +4 to balance. Lead commonly shows +2 or +4, so you need the Roman numeral: Lead(IV). Sulfate keeps its name. No ide suffix for polyatomic ions.
Problem 6: Write the Formula
Iron(III) hydroxide
Answer: Fe(OH)₃
Explanation: Iron(III) is Fe³⁺. Hydroxide is OH⁻. You need three hydroxide ions to balance one iron(III) ion: +3 + (-1 × 3) = 0. The hydroxide polyatomic ion needs parentheses when subscripted: Fe(OH)₃.
Problem 7: Name This Compound
MnO₂
Answer: Manganese(IV) oxide
Explanation: Mn is manganese, a transition metal with multiple possible charges. O is oxygen, -2. Two oxygens give -4 total. Manganese must be +4. Manganese(IV) oxide. Common student mistake: writing manganese dioxide. Dioxide is just a descriptor meaning two oxygens—the systematic name is manganese(IV) oxide.
Problem 8: Write the Formula
Zinc phosphate
Answer: Zn₃(PO₄)₂
Explanation: Zinc is Zn²⁺ (Group 12, fixed charge). Phosphate is PO₄³⁻. Find the least common multiple: 6. You need 3 zinc ions (3 × +2 = +6) and 2 phosphate ions (2 × -3 = -6). Result: Zn₃(PO₄)₂.
Quick Reference: Common Polyatomic Ions
These show up constantly. Memorize them or lose points.
| Ion | Charge | Common Mistake |
|---|---|---|
| Nitrate (NO₃⁻) | -1 | Writing "nitrite" |
| Nitrite (NO₂⁻) | -1 | Confusing with nitrate |
| Sulfate (SO₄²⁻) | -2 | Forgetting the charge |
| Hydroxide (OH⁻) | -1 | Writing "hydroxide" as "hydride" |
| Carbonate (CO₃²⁻) | -2 | Forgetting parentheses in formulas |
| Ammonium (NH₄⁺) | +1 | Treating it like a metal |
| Phosphate (PO₄³⁻) | -3 | Using wrong subscript combinations |
Common Mistakes That Kill Your Grade
1. Forgetting parentheses: When polyatomic ions need subscripts, wrap them first. Fe(OH)₃, not FeOH₃. The parentheses tell you the whole group is multiplied.
2. Skipping Roman numerals: If the metal can have multiple charges, you must include the charge in parentheses. FeCl₂ is iron(II) chloride. FeCl₃ is iron(III) chloride. Different compounds, different properties.
3. Guessing charges on transition metals: Calculate from the nonmetal or polyatomic ion. Don't guess based on position on the periodic table. Use the anion to determine the metal's charge, not the other way around.
4. Dropping the -ide suffix: Simple anions always end in -ide. Sulfide, chloride, oxide, nitride. If you're naming something with a polyatomic ion, don't add -ide. Sulfate isn't sulfide.
5. Mixing up similar names: Nitrate vs. nitrite. Sulfate vs. sulfite. Phosphate vs. phosphite. Different formulas, different compounds. One extra oxygen makes a completely different ion.
How to Approach Any Mixed Problem
Step 1: Identify the cation. Is it a simple metal (Group 1, 2, 13), transition metal, or ammonium?
Step 2: Identify the anion. Simple nonmetal (ends in -ide) or polyatomic ion?
Step 3: Determine charges. Use known charges for simple ions. For variable-charge metals, calculate from the anion's charge.
Step 4: Balance the formula. Find the least common multiple of the charges. Write the formula with correct subscripts and parentheses.
Step 5: Name it. Cation name first. Add Roman numeral if needed. Anion name second.
That's the entire process. Practice it until it's automatic.
When to Ask for Help
If you're still missing problems after working through this set, the issue is usually one of two things:
- You don't know the polyatomic ion charges. Fix this by drilling flashcards until they're instant.
- You're not calculating charges correctly. Go back to the LCM method and practice with simpler compounds first.
Mixed ionic compound problems aren't hard once the patterns click. They just require knowing your ions and applying the same logic to every problem. No exceptions, no shortcuts.