AP Chemistry Quiz: Representations Of Reactions

What this quiz covers

This quiz focuses on Representations Of Reactions, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. CaCl$_2$(aq) + Na$_2$CO$_3$(aq)  CaCO$_3$(aq) + 2NaCl(s)
2. CaCl$_2$(aq) + Na$_2$CO$_3$(aq)  CaCO$_3$(s) + 2NaCl(aq) (correct answer)
3. CaCl$_2$(aq) + Na$_2$CO$_3$(aq)  Ca$_2$CO$_3$(s) + 2NaCl(aq)
4. CaCl$_2$(aq) + NaCO$_3$(aq)  CaCO$_3$(s) + NaCl(aq)
5. CaCl$_2$(s) + Na$_2$CO$_3$(aq)  CaCO$_3$(s) + 2NaCl(aq)

Explanation: This question assesses the skill of representations of reactions. To check consistency across representations, first count the atoms of each element on both sides of the equation to ensure conservation, such as 1 Ca, 2 Cl, 2 Na, 1 C, and 3 O in the balanced form. Next, verify that charges are conserved if ionic species are present, though this molecular equation shows neutral compounds. Additionally, confirm that the states of matter align with the description, like the white precipitate being solid CaCO3. A tempting distractor is choice A, which incorrectly shows CaCO3 as aqueous and NaCl as solid, reversing the solubility rules for these compounds. Always check atom count and charge before worrying about reaction type.

Question 2

1. MgCl$_2$(s) + 2AgNO$_3$(aq)  2AgCl(aq) + Mg(NO$_3$)$_2$(aq)
2. MgCl$_2$(aq) + 2AgNO$_3$(aq)  2AgCl(s) + Mg(NO$_3$)$_2$(aq) (correct answer)
3. MgCl$_2$(aq) + AgNO$_3$(aq)  AgCl(s) + MgNO$_3$(aq)
4. MgCl$_2$(aq) + 2AgNO$_3$(aq)  2AgCl(s) + MgNO$_3$(aq)
5. Mg$^{2+}$(aq) + 2Cl$^-$(aq) + 2AgNO$_3$(aq)  2AgCl(s) + Mg$^{2+}$(aq) + 2NO$_3^-$(aq)

Explanation: This question assesses the skill of representations of reactions. To check consistency across representations, count atoms like 1 Mg, 2 Cl, 2 Ag, 2 N, 6 O on both sides. Verify neutral charges in this molecular equation. Ensure states match: aqueous reactants to solid AgCl and aqueous Mg(NO3)2. A tempting distractor is choice A, which lacks coefficients, unbalancing Cl and Ag atoms. Always check atom count and charge before worrying about reaction type.

Question 3

1. $\mathrm{Na^+(aq) + Cl^-(aq) \rightarrow NaCl(aq)}$
2. $\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}$
3. $\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$ (correct answer)
4. $\mathrm{H^+(aq) + Cl^-(aq) + Na^+(aq) + OH^-(aq) \rightarrow NaCl(s) + H_2O(l)}$
5. $\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2(g) + O^{2-}(aq)}$

Explanation: This question tests the skill of representations of reactions by asking you to convert a molecular equation to its net ionic form. To verify the net ionic equation, first write the complete ionic form: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l). Then identify spectator ions (Na⁺ and Cl⁻ appear unchanged on both sides) and remove them. Choice A incorrectly shows the formation of NaCl from its ions, which are actually spectators. The correct answer C shows only the reacting species: H⁺(aq) + OH⁻(aq) → H₂O(l), representing the essential acid-base neutralization. To write net ionic equations, include only the species that undergo chemical change and exclude all spectator ions.

Question 4

1. $\mathrm{MgO(s) + 2\,HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)}$ (correct answer)
2. $\mathrm{MgO(aq) + 2\,HCl(aq) \rightarrow MgCl_2(s) + H_2O(aq)}$
3. $\mathrm{MgO(s) + HCl(aq) \rightarrow MgCl(aq) + HOH(l)}$
4. $\mathrm{Mg(s) + 2\,HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)}$
5. $\mathrm{MgO(s) + H_2O(l) \rightarrow Mg(OH)_2(aq)}$

Explanation: This problem requires the skill of representations of reactions to write a balanced equation matching a verbal description. Checking conservation: Mg (1→1), O (1→1), H (2→2), Cl (2→2), all atoms balance correctly. The equation also shows correct physical states: MgO is a solid metal oxide, HCl is aqueous acid, MgCl₂ is soluble (aqueous), and water is liquid. Choice D shows a different reaction—metallic Mg reacting with HCl to produce H₂ gas, not the acid-base reaction described. Always match the chemical species and physical states exactly to the problem description.

Question 5

1. Mg(s) + H$^+$(aq)  Mg$^{2+}$(aq) + H(g)
2. Mg(s) + 2H$^+$(aq)  Mg$^{2+}$(aq) + H$_2$(g) (correct answer)
3. Mg$^{2+}$(aq) + 2H$^+$(aq)  Mg(s) + H$_2$(g)
4. Mg(s) + H$_2$(aq)  Mg$^{2+}$(aq) + 2H$^-$(aq)
5. 2Mg(s) + 2H$^+$(aq)  2Mg$^{2+}$(aq) + H$_2$(g)

Explanation: This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: 1 Mg, 2 H on both sides. Verify charge: +2 on left (2H+) matches +2 on right (Mg2+). Confirm states: solid Mg, aqueous H+ to aqueous Mg2+, gaseous H2. A tempting distractor is choice A, which omits the coefficient 2 for H+, unbalancing atoms and charge. Always check atom count and charge before worrying about reaction type.

Question 6

1. $\mathrm{NH_4Cl(aq) + NaOH(aq) \rightarrow NH_3(aq) + H_2O(l) + NaCl(aq)}$ (correct answer)
2. $\mathrm{NH_4Cl(s) + NaOH(aq) \rightarrow NH_4OH(aq) + NaCl(s)}$
3. $\mathrm{NH_4Cl(aq) + NaOH(aq) \rightarrow NH_4OH(aq) + NaCl(aq)}$
4. $\mathrm{NH_3(aq) + HCl(aq) \rightarrow NH_4Cl(aq)}$
5. $\mathrm{NH_4Cl(aq) + NaOH(aq) \rightarrow NH_2Cl(aq) + NaH_2O(aq)}$

Explanation: This question tests the skill of representations of reactions by asking you to match a net ionic equation with its molecular form. To verify consistency, the molecular equation must produce the same net ionic equation when spectator ions are removed: NH₄Cl provides NH₄⁺ and Cl⁻, NaOH provides Na⁺ and OH⁻, and after removing spectators (Na⁺ and Cl⁻), we get NH₄⁺ + OH⁻ → NH₃ + H₂O. Choice C incorrectly shows NH₄OH as a product, but this compound doesn't exist as a stable molecule (ammonia exists as NH₃ in water). The correct answer A shows NH₄Cl(aq) + NaOH(aq) → NH₃(aq) + H₂O(l) + NaCl(aq), which yields the given net ionic equation when spectator ions are removed. To verify molecular equations match net ionic equations, write the complete ionic form and confirm that removing spectators gives the target net equation.

Question 7

1. $\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$ (correct answer)
2. $\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(aq) + NaNO_3(s)}$
3. $\mathrm{AgNO_3(s) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$
4. $\mathrm{AgCl(aq) \rightarrow Ag^+(aq) + Cl^-(aq)}$
5. $\mathrm{AgNO_3(aq) + HCl(aq) \rightarrow AgCl(s) + HNO_3(s)}$

Explanation: This problem tests representations of reactions by connecting a particulate description to a complete molecular equation. The equation must show all species including spectator ions (Na⁺ and NO₃⁻) that don't participate in the net reaction. Verifying conservation: Ag (1→1), N (1→1), O (3→3), Na (1→1), Cl (1→1), all atoms balance. Choice D shows only a dissolution equation, missing the precipitation reaction entirely. When converting between net ionic and molecular equations, always include spectator ions and verify physical states match the description.

Question 8

1. $\mathrm{Na_2CO_3(aq) + 2\,HCl(aq) \rightarrow 2\,NaCl(aq) + H_2O(l) + CO_2(g)}$
2. $\mathrm{CO_3^{2-}(aq) + 2\,H^+(aq) \rightarrow H_2O(l) + CO_2(g)}$ (correct answer)
3. $\mathrm{2\,Na^+(aq) + 2\,Cl^-(aq) \rightarrow 2\,NaCl(s)}$
4. $\mathrm{CO_2(g) + H_2O(l) \rightarrow CO_3^{2-}(aq) + 2\,H^+(aq)}$
5. $\mathrm{CO_3^{2-}(aq) + 2\,HCl(aq) \rightarrow CO_2(g) + H_2O(l) + 2\,Cl^-(aq)}$

Explanation: This question requires the skill of representations of reactions to identify the net ionic equation from a complete ionic equation. The complete ionic equation shows Na⁺ and Cl⁻ appearing unchanged on both sides—these are spectator ions to be removed. The net ionic equation shows only: CO₃²⁻ + 2H⁺ → H₂O + CO₂. Verifying conservation: C (1→1), O (3→3), H (2→2), charge (-2 + 2(+1) = 0 on both sides). Choice E incorrectly includes Cl⁻ ions, which are spectators. To find net ionic equations, cancel all ions that appear identically on both sides of the complete ionic equation.

Question 9

1. $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(s) + 2\,NaCl(aq)}$ (correct answer)
2. $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(aq) + 2\,NaCl(s)}$
3. $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow Ca_2CO_3(s) + 2\,NaCl(aq)}$
4. $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCl(s) + Na_2CO_3(aq)}$
5. $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(s) + NaCl(aq)}$

Explanation: This question tests the skill of representations of reactions by asking you to identify the correct molecular equation for a precipitation reaction. To verify the equation is balanced, count atoms on each side: Ca (1→1), Cl (2→2), Na (2→2), C (1→1), O (3→3), all balanced. The charge is also conserved since all compounds are neutral. Choice B incorrectly shows NaCl as a solid when sodium chloride is highly soluble in water. Always verify that precipitates match known solubility rules—carbonates of Group 2 metals like CaCO₃ are insoluble while alkali metal salts like NaCl are soluble.

Question 10

1. $\mathrm{Al^{3+}(aq) + PO_4^{3-}(aq) \rightarrow AlPO_4(s)}$ (correct answer)
2. $\mathrm{Al^{3+}(aq) + PO_4^{3-}(aq) \rightarrow AlPO_4(aq)}$
3. $\mathrm{Al^{2+}(aq) + PO_4^{3-}(aq) \rightarrow AlPO_4(s)}$
4. $\mathrm{Al^{3+}(aq) + 3\,PO_4^-(aq) \rightarrow Al(PO_4)_3(s)}$
5. $\mathrm{Al^{3+}(aq) + PO_4^{3-}(aq) \rightarrow Al(s) + PO_4(aq)}$

Explanation: This problem tests representations of reactions for a precipitation reaction at the ionic level. The equation must balance atoms and charge: Al (1→1), P (1→1), O (4→4), with charge +3 + (-3) = 0 on the left and 0 on the right. AlPO₄ forms as a solid precipitate when Al³⁺ and PO₄³⁻ ions combine in aqueous solution. Choice C incorrectly shows Al²⁺, but aluminum forms +3 ions, not +2. When writing ionic equations, always use the correct charges for ions based on their position in the periodic table or polyatomic ion charges.

Question 11

1. $\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$ (correct answer)
2. $\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2(g) + O^{2-}(aq)}$
3. $\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(aq)}$
4. $\mathrm{2\,H^+(aq) + O^{2-}(aq) \rightarrow H_2O(l)}$
5. $\mathrm{H^+(aq) + OH^-(aq) \rightarrow HOH^+(aq)}$

Explanation: This question tests representations of reactions for an acid-base neutralization at the ionic level. The equation must conserve both atoms and charge: left side has 1 H, 1 O, and net charge of 0 (+1 + -1); right side has 2 H, 1 O in H₂O(l). Charge balances (0→0) and atoms balance when water is written as H₂O. Choice B incorrectly shows products as H₂(g) and O²⁻, which neither conserves atoms nor represents the actual neutralization product. For acid-base reactions, remember that H⁺ + OH⁻ always produces liquid water, not aqueous water or other species.

Question 12

1. $\mathrm{H^+(aq) + CO_3^{2-}(aq) \rightarrow HCO_3^-(aq)}$
2. $\mathrm{2\,H^+(aq) + CO_3^{2-}(aq) \rightarrow CO_2(g) + H_2O(l)}$ (correct answer)
3. $\mathrm{2\,H^+(aq) + CO_3^{2-}(aq) \rightarrow CO(g) + H_2O(l) + O_2(g)}$
4. $\mathrm{H^+(aq) + CO_3^{2-}(aq) \rightarrow CO_2(g) + OH^-(aq)}$
5. $\mathrm{2\,H^+(aq) + 2\,CO_3^{2-}(aq) \rightarrow 2\,CO_2(g) + H_2O(l)}$

Explanation: This question tests the skill of representations of reactions by asking you to write an equation from a particle-level description. To verify the equation matches the description, check that exactly 2 H⁺ ions react with exactly 1 CO₃²⁻ ion to form exactly 1 CO₂ and 1 H₂O molecule. Also verify charge balance: reactants have 2(+1) + 1(-2) = 0, and products are neutral molecules with net charge 0. Choice E incorrectly shows 2 CO₃²⁻ ions reacting, which doesn't match the description of one carbonate ion. The correct answer B shows 2 H⁺(aq) + CO₃²⁻(aq) → CO₂(g) + H₂O(l), exactly matching the particle-level description with proper stoichiometry. When converting particle descriptions to equations, ensure the coefficients exactly match the number of each particle type described.

Question 13

1. $\mathrm{Ba^{2+}(aq) + 2\,NO_3^-(aq) + 2\,Na^+(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s) + 2\,Na^+(aq) + 2\,NO_3^-(aq)}$ (correct answer)
2. $\mathrm{Ba^{2+}(aq) + NO_3^-(aq) + Na^+(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s) + NaNO_3(aq)}$
3. $\mathrm{Ba(NO_3)_2(aq) + 2\,Na^+(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s) + 2\,NaNO_3(aq)}$
4. $\mathrm{Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)}$
5. $\mathrm{Ba^{2+}(aq) + 2\,NO_3^-(aq) + 2\,Na^+(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(aq) + 2\,Na^+(aq) + 2\,NO_3^-(aq)}$

Explanation: This question tests the skill of representations of reactions by asking for a complete ionic equation that includes spectator ions. To verify consistency, check that all atoms are conserved: Ba, N, O, Na, and S atoms must be equal on both sides. Also verify charge balance: left side has Ba²⁺, 2 NO₃⁻, 2 Na⁺, and SO₄²⁻ (net = 0), and right side must also equal 0. Choice D shows only the net ionic equation, omitting the required spectator ions Na⁺ and NO₃⁻. The correct answer A properly shows all ions before and after reaction: Ba²⁺(aq) + 2 NO₃⁻(aq) + 2 Na⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2 Na⁺(aq) + 2 NO₃⁻(aq), with BaSO₄ correctly shown as solid. When writing complete ionic equations, include all dissolved ions and show spectator ions unchanged on both sides.

Question 14

1. H$_2$(g) + Cl$_2$(g)  2HCl(g) (correct answer)
2. 2H$_2$(g) + Cl$_2$(g)  2HCl(g)
3. H$_2$(g) + 2Cl$_2$(g)  2HCl(g)
4. HCl(g)  H$_2$(g) + Cl$_2$(g)
5. 2H$_2$(g) + 2Cl$_2$(g)  HCl(g)

Explanation: This question assesses the skill of representations of reactions. To check consistency across representations, count the atoms in the particulate diagram and ensure the equation matches, with equal H and Cl on both sides. Verify that charges are neutral in this molecular equation, as all are gases. Confirm the coefficients reflect the diatomic nature, like 1 H2, 1 Cl2 to 2 HCl. A tempting distractor is choice B, which doubles H2 but not Cl2, violating atom conservation. Always check atom count and charge before worrying about reaction type.

Question 15

1. $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(aq) + 2\,NaCl(s)}$
2. $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(s) + 2\,NaCl(aq)}$ (correct answer)
3. $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(s) + NaCl(aq)}$
4. $\mathrm{CaCl_2(s) + Na_2CO_3(aq) \rightarrow CaCO_3(s) + 2\,NaCl(aq)}$
5. $\mathrm{CaCl_2(aq) + Na_2CO_3(aq) \rightarrow Ca(s) + Na_2Cl_2(aq) + CO_3(aq)}$

Explanation: This question tests the skill of representations of reactions by asking you to identify the correct molecular equation with proper physical states. To check consistency across representations, verify that atoms are conserved: reactants have 1 Ca, 2 Cl, 2 Na, 1 C, and 3 O atoms, which must equal the products. Also verify charge is balanced: all species are neutral compounds, so charge is automatically balanced. Choice A incorrectly shows NaCl as a solid when it should be aqueous (NaCl is highly soluble), and CaCO₃ as aqueous when it should be solid (CaCO₃ is insoluble). The correct answer B properly shows CaCO₃(s) as the precipitate and NaCl(aq) remaining dissolved, with a coefficient of 2 for NaCl to balance the equation. When analyzing reaction representations, always verify both atom conservation and correct physical states based on solubility rules.

Question 16

1. NaCl(aq) + AgNO$_3$(aq)  AgCl(s) + NaNO$_3$(aq) (correct answer)
2. NaCl(s) + AgNO$_3$(aq)  AgCl(aq) + NaNO$_3$(aq)
3. Na$^+$(aq) + Cl$^-$(aq) + Ag$^+$(aq) + NO$_3^-$(aq)  AgCl(s) + NaNO$_3$(s)
4. NaCl(aq) + AgNO$_3$(aq)  NaAgClNO$_3$(s)
5. 2NaCl(aq) + AgNO$_3$(aq)  AgCl(s) + NaNO$_3$(aq) + NaCl(aq)

Explanation: This question assesses the skill of representations of reactions. To check consistency across representations, count atoms from the diagram: 1 Na, 1 Cl, 1 Ag, 1 N, 3 O on both sides. Verify neutral charges in molecular form. Ensure states: aqueous to solid AgCl, aqueous NaNO3. A tempting distractor is choice C, which shows complete ionic but incorrectly has NaNO3 as solid. Always check atom count and charge before worrying about reaction type.

Question 17

1. $\mathrm{KClO_3(s)} \rightarrow \mathrm{KCl(s)} + \mathrm{O_2(g)}$
2. $\mathrm{2KClO_3(s)} \rightarrow \mathrm{2KCl(s)} + \mathrm{3O_2(g)}$ (correct answer)
3. $\mathrm{K_2ClO_3(s)} \rightarrow \mathrm{2KCl(s)} + \mathrm{O_2(g)}$
4. $\mathrm{2KClO_3(s)} \rightarrow \mathrm{KCl(s)} + \mathrm{3O_2(g)}$
5. $\mathrm{KClO_3(s)} \rightarrow \mathrm{KCl(s)} + \mathrm{3O(g)}$

Explanation: This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: $2 \mathrm{K}$, $2 \mathrm{Cl}$, $6 \mathrm{O}$ on left to $2 \mathrm{K}$, $2 \mathrm{Cl}$, $6 \mathrm{O}$ on right. Verify neutral charges. Ensure coefficients balance oxygen: $2 \mathrm{KClO_3}$ to $2 \mathrm{KCl} + 3 \mathrm{O_2}$. A tempting distractor is choice A, lacking coefficients, unbalancing oxygen atoms. Always check atom count and charge before worrying about reaction type.

Question 18

1. 2Al(s) + 3O$_2$(g)  2Al$_2$O$_3$(s)
2. 2Al(s) + O$_2$(g)  Al$_2$O$_3$(s)
3. 3Al(s) + 2O$_2$(g)  Al$_3$O$_4$(s)
4. 4Al(s) + 3O$_2$(g)  2Al$_2$O$_3$(s) (correct answer)
5. Al(s) + 3O(g)  AlO$_3$(s)

Explanation: This question assesses the skill of representations of reactions. To check consistency across representations, count atoms: 4 Al, 6 O on left to 4 Al, 6 O on right. Verify neutral charges. Ensure coefficients balance oxygen: 4 Al + 3 O2 to 2 Al2O3. A tempting distractor is choice A, with unbalanced coefficients for Al and O2. Always check atom count and charge before worrying about reaction type.

Question 19

1. Pb$^{2+}$(aq) + 2NO$_3^-$(aq)  Pb(NO$_3$)$_2$(s)
2. K$^+$(aq) + I$^-$(aq)  KI(s)
3. Pb$^{2+}$(aq) + 2I$^-$(aq)  PbI$_2$(s) (correct answer)
4. Pb(NO$_3$)$_2$(aq) + 2KI(aq)  PbI$_2$(s) + 2KNO$_3$(aq)
5. Pb$^{2+}$(aq) + 2K$^+$(aq)  PbK$_2$(aq)

Explanation: This question assesses the skill of representations of reactions. To check consistency across representations, count atoms from the molecular equation: 1 Pb, 2 N, 6 O, 2 K, 2 I on both sides. Verify charge in net ionic: +2 (Pb) and -2 (2I) to neutral PbI2. Remove spectators like K+ and NO3- for the net form. A tempting distractor is choice D, which is the molecular equation, not net ionic. Always check atom count and charge before worrying about reaction type.

Question 20

1. $\mathrm{PbI_2(aq) \rightarrow Pb^{2+}(aq) + 2\,I^-(aq)}$
2. $\mathrm{Pb(NO_3)_2(aq) + 2\,KI(aq) \rightarrow PbI_2(s) + 2\,KNO_3(aq)}$ (correct answer)
3. $\mathrm{Pb(NO_3)_2(aq) + KI(aq) \rightarrow PbI_2(s) + KNO_3(aq)}$
4. $\mathrm{PbI_2(s) + 2\,KNO_3(aq) \rightarrow Pb(NO_3)_2(aq) + 2\,KI(aq)}$
5. $\mathrm{Pb(NO_3)_2(aq) + 2\,KI(s) \rightarrow PbI_2(s) + 2\,KNO_3(aq)}$

Explanation: This question tests representations of reactions by asking you to match a net ionic equation with its corresponding molecular equation. The net ionic equation shows Pb²⁺ + 2I⁻ → PbI₂(s), so the molecular equation must provide these ions from soluble compounds. Checking atom balance: Pb (1→1), N (2→2), O (6→6), K (2→2), I (2→2), all balanced correctly. Choice C fails to balance iodine atoms (only 1 I on the left but 2 I in PbI₂). Always verify that coefficients in the molecular equation produce the correct number of ions shown in the net ionic equation.