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Portland State University
● ●
PSU Department of Chemistry — CHEM 210 (Patel) Exam 2
LET KNOWLEDGE SERVE THE CITY
CHEM 210
CHEM 210 — Examination 2 (Patel)
POLYATOMIC IONS, ACID NOMENCLATURE & CHEMICAL FORMULAS | 2026/2027
INSTITUTION Portland State University (PSU) COURSE CODE CHEM 210 — Instructor: Patel
PROGRAM Undergraduate Chemistry / Pre-Professional ACADEMIC YEAR
EXAM TITLE Examination 2 — Polyatomic Ions & Acids TOTAL QUESTIONS 24 Questions
SUBJECT AREAS Polyatomic Ions, Oxyanions, Acid Nomenclature, FORMAT Multiple Choice — Select the Single Best Answer
Strength
EXAMINATION INSTRUCTIONS
▸ Select the single best answer for each multiple-choice question.
▸ Memorize the names, formulas, and charges of all common polyatomic ions (sulfate, sulfite, nitrate, nitrite, phosphate, phosphite, carbonate, hydroxide, ammonium,
dihydrogen phosphate, permanganate).
▸ Know the names and formulas of binary acids (hydrohalic acids) and oxyacids, and distinguish strong acids from weak acids.
▸ Strong acids: HCl, HBr, HI, HNO3, H2SO4, HClO4. All others are weak acids.
▸ Correct answers and detailed rationales appear below each question for exam review purposes.
SECTION I — POLYATOMIC IONS: NAMES, FORMULAS & CHARGES Questions 1 – 14
1. What is the correct formula for the sulfate ion?
A. SO₃⁻²
B. SO₄⁻²
C. SO₄⁻¹
D. S₂O₃⁻²
CORRECT ANSWER B — SO₄⁻²; sulfate has four oxygen atoms and a −2 charge
RATIONALE Sulfate (SO₄⁻²) is one of the most important polyatomic ions. Key distinguishing feature: the "-ate" ending indicates MORE oxygen atoms than the "-
ite" form. SulfATE = 4 oxygens (SO₄⁻²). SulfITE = 3 oxygens (SO₃⁻²). Both carry a −2 charge. This pattern is consistent across the sulfate/sulfite,
nitrate/nitrite, and phosphate/phosphite families. The mnemonic: "-ate" has more oxygen atoms (the "A" in "ate" = "A" lot). Option A is sulfite.
Option C has the wrong charge. Option D is thiosulfate (one oxygen replaced by sulfur).
2. What is the correct formula for the sulfite ion?
A. SO₃⁻²
B. SO₄⁻²
C. SO₃⁻¹
D. HSO₃⁻¹
CORRECT ANSWER A — SO₃⁻²; sulfite has THREE oxygen atoms and a −2 charge
RATIONALE Sulfite (SO₃⁻²) has one fewer oxygen atom than sulfate (SO₄⁻²). The "-ite" ending indicates FEWER oxygen atoms than the corresponding "-ate" ion.
Both sulfate and sulfite carry a −2 charge. The charge does NOT change between the -ate and -ite forms — only the number of oxygen atoms
changes. Option B is sulfate (4 oxygens). Option C has the wrong charge. Option D is the hydrogen sulfite (bisulfite) ion, which has an added H⁺ and
a −1 charge.
3. What is the difference between sulfate (SO₄⁻²) and sulfite (SO₃⁻²)?
A. Sulfate has a −1 charge; sulfite has a −2 charge
B. Sulfate has 4 oxygen atoms; sulfite has 3 oxygen atoms — both have −2 charge; the "-ate" ion has one more oxygen than the "-ite" ion
C. Sulfate has 3 oxygen atoms; sulfite has 4 oxygen atoms
D. They are identical in formula but differ in charge
CORRECT ANSWER B — SulfATE (4 O) vs. SulfITE (3 O); both −2 charge; the "-ate" form always has one MORE oxygen than "-ite"
RATIONALE The -ate/-ite naming convention is systematic across polyatomic oxyanions: the "-ate" ion has MORE oxygen atoms; the "-ite" ion has one FEWER
oxygen. The charge is the SAME for both. Examples: NitrATE NO₃⁻ vs. NitrITE NO₂⁻ (both −1). PhosphATE PO₄⁻³ vs. PhosphITE PO₃⁻³ (both −3). This
pattern is essential for naming oxyacids: -ate → -ic acid (H₂SO₄ = sulfuric acid), -ite → -ous acid (H₂SO₃ = sulfurous acid).
, 4. What is the formula of the nitrate ion?
A. NO₂⁻¹
B. NO₃⁻¹
C. NO₃⁻²
D. NH₄⁺
CORRECT ANSWER B — NO₃⁻¹; nitrate has three oxygen atoms and a −1 charge
RATIONALE Nitrate (NO₃⁻) has three oxygen atoms and a −1 charge. Nitrite (NO₂⁻, option A) has two oxygens and −1 charge. Both have the SAME charge (−1) —
the difference is only in the number of oxygens. Nitrate is the conjugate base of nitric acid (HNO₃, a strong acid). Option C has the wrong charge.
Option D is the ammonium cation (NH₄⁺), a polyatomic cation — not an oxyanion of nitrogen. The NO₃⁻ ion is trigonal planar with resonance-
stabilized N-O bonds.
5. What is the formula of the nitrite ion?
A. NO₃⁻¹
B. NO₂⁻¹
C. N₂O₄⁻²
D. NO⁻¹
CORRECT ANSWER B — NO₂⁻¹; nitrite has two oxygen atoms and a −1 charge
RATIONALE Nitrite (NO₂⁻) has one fewer oxygen than nitrate (NO₃⁻), following the -ate/-ite pattern. Both have −1 charge. Nitrite is the conjugate base of nitrous
acid (HNO₂, a weak acid). The nitrite ion has a bent molecular geometry with resonance between the two N-O bonds. Option A is nitrate (3 oxygens).
Option C is dinitrogen tetroxide, a neutral molecule. Option D is the hypothetical NO⁻ which is not a stable polyatomic ion.
6. What is the formula of the phosphate ion?
A. PO₃⁻³
B. PO₄⁻³
C. PO₄⁻²
D. HPO₄⁻²
CORRECT ANSWER B — PO₄⁻³; phosphate has four oxygen atoms and a −3 charge
RATIONALE Phosphate (PO₄⁻³) is a key biological and geological polyatomic ion. It has four oxygen atoms and a −3 charge. PhosphITE (PO₃⁻³, option A) has three
oxygens and the SAME −3 charge. Both follow the -ate/-ite pattern with identical charges. Option C has the wrong charge (−2 instead of −3). Option D
is the hydrogen phosphate ion (HPO₄⁻²), where one H⁺ is added to phosphate, reducing the charge by one. Phosphate is tetrahedral and is the
conjugate base of phosphoric acid (H₃PO₄).
7. What is the formula of the carbonate ion?
A. CO₂⁻²
B. CO₃⁻²
C. HCO₃⁻¹
D. CO₃⁻¹
CORRECT ANSWER B — CO₃⁻²; carbonate has three oxygen atoms and a −2 charge; it is trigonal planar with resonance
RATIONALE Carbonate (CO₃⁻²) has three oxygen atoms arranged in a trigonal planar geometry around the central carbon, with resonance delocalization of the
−2 charge. Option A (CO₂⁻²) is not a stable ion — CO₂ is carbon dioxide (neutral molecule). Option C (HCO₃⁻) is hydrogen carbonate (bicarbonate).
Option D has the wrong charge. Carbonate is found in limestone (CaCO₃), baking soda (NaHCO₃), and is the conjugate base of carbonic acid (H₂CO₃).
The carbonate ion is a classic example of resonance stabilization.
8. What is the formula of the hydroxide ion?
A. H₃O⁺
B. OH⁻¹
C. O⁻²
D. HO₂⁻¹
CORRECT ANSWER B — OH⁻¹; hydroxide has one oxygen, one hydrogen, and a −1 charge
RATIONALE Hydroxide (OH⁻) is one of the most fundamental polyatomic ions — it is the defining species of basic solutions. Arrhenius bases dissociate to
produce OH⁻ in water. Option A (H₃O⁺) is the hydronium ion — the conjugate acid of water, not hydroxide. Option C (O⁻²) is the oxide ion, a
monatomic anion. Option D (HO₂⁻) is the hydrogen peroxide anion. Hydroxide forms strong bases with Group 1 and 2 metals (NaOH, KOH, Ca(OH)₂).
9. What is the formula of the ammonium ion?
A. NH₃
B. NH₄⁺¹
C. NO₃⁻
D. NH₂⁻
CORRECT ANSWER B — NH₄⁺; ammonium is a POLYCATION (positively charged polyatomic ion) with a +1 charge
RATIONALE Ammonium (NH₄⁺) is one of the few common polyatomic CATIONS. It is formed by protonation of ammonia (NH₃ + H⁺ → NH₄⁺). It has a tetrahedral
geometry (sp3 hybridized nitrogen). Option A is ammonia (NH₃), the neutral molecule. Option C is nitrate (NO₃⁻), an anion. Option D is the amide ion
(NH₂⁻). Ammonium salts (NH₄Cl, NH₄NO₃) are common in fertilizers. The ammonium ion has a pKa of approximately 9.25, making it a weak acid.
Most other polyatomic ions are anions.
Portland State University
● ●
PSU Department of Chemistry — CHEM 210 (Patel) Exam 2
LET KNOWLEDGE SERVE THE CITY
CHEM 210
CHEM 210 — Examination 2 (Patel)
POLYATOMIC IONS, ACID NOMENCLATURE & CHEMICAL FORMULAS | 2026/2027
INSTITUTION Portland State University (PSU) COURSE CODE CHEM 210 — Instructor: Patel
PROGRAM Undergraduate Chemistry / Pre-Professional ACADEMIC YEAR
EXAM TITLE Examination 2 — Polyatomic Ions & Acids TOTAL QUESTIONS 24 Questions
SUBJECT AREAS Polyatomic Ions, Oxyanions, Acid Nomenclature, FORMAT Multiple Choice — Select the Single Best Answer
Strength
EXAMINATION INSTRUCTIONS
▸ Select the single best answer for each multiple-choice question.
▸ Memorize the names, formulas, and charges of all common polyatomic ions (sulfate, sulfite, nitrate, nitrite, phosphate, phosphite, carbonate, hydroxide, ammonium,
dihydrogen phosphate, permanganate).
▸ Know the names and formulas of binary acids (hydrohalic acids) and oxyacids, and distinguish strong acids from weak acids.
▸ Strong acids: HCl, HBr, HI, HNO3, H2SO4, HClO4. All others are weak acids.
▸ Correct answers and detailed rationales appear below each question for exam review purposes.
SECTION I — POLYATOMIC IONS: NAMES, FORMULAS & CHARGES Questions 1 – 14
1. What is the correct formula for the sulfate ion?
A. SO₃⁻²
B. SO₄⁻²
C. SO₄⁻¹
D. S₂O₃⁻²
CORRECT ANSWER B — SO₄⁻²; sulfate has four oxygen atoms and a −2 charge
RATIONALE Sulfate (SO₄⁻²) is one of the most important polyatomic ions. Key distinguishing feature: the "-ate" ending indicates MORE oxygen atoms than the "-
ite" form. SulfATE = 4 oxygens (SO₄⁻²). SulfITE = 3 oxygens (SO₃⁻²). Both carry a −2 charge. This pattern is consistent across the sulfate/sulfite,
nitrate/nitrite, and phosphate/phosphite families. The mnemonic: "-ate" has more oxygen atoms (the "A" in "ate" = "A" lot). Option A is sulfite.
Option C has the wrong charge. Option D is thiosulfate (one oxygen replaced by sulfur).
2. What is the correct formula for the sulfite ion?
A. SO₃⁻²
B. SO₄⁻²
C. SO₃⁻¹
D. HSO₃⁻¹
CORRECT ANSWER A — SO₃⁻²; sulfite has THREE oxygen atoms and a −2 charge
RATIONALE Sulfite (SO₃⁻²) has one fewer oxygen atom than sulfate (SO₄⁻²). The "-ite" ending indicates FEWER oxygen atoms than the corresponding "-ate" ion.
Both sulfate and sulfite carry a −2 charge. The charge does NOT change between the -ate and -ite forms — only the number of oxygen atoms
changes. Option B is sulfate (4 oxygens). Option C has the wrong charge. Option D is the hydrogen sulfite (bisulfite) ion, which has an added H⁺ and
a −1 charge.
3. What is the difference between sulfate (SO₄⁻²) and sulfite (SO₃⁻²)?
A. Sulfate has a −1 charge; sulfite has a −2 charge
B. Sulfate has 4 oxygen atoms; sulfite has 3 oxygen atoms — both have −2 charge; the "-ate" ion has one more oxygen than the "-ite" ion
C. Sulfate has 3 oxygen atoms; sulfite has 4 oxygen atoms
D. They are identical in formula but differ in charge
CORRECT ANSWER B — SulfATE (4 O) vs. SulfITE (3 O); both −2 charge; the "-ate" form always has one MORE oxygen than "-ite"
RATIONALE The -ate/-ite naming convention is systematic across polyatomic oxyanions: the "-ate" ion has MORE oxygen atoms; the "-ite" ion has one FEWER
oxygen. The charge is the SAME for both. Examples: NitrATE NO₃⁻ vs. NitrITE NO₂⁻ (both −1). PhosphATE PO₄⁻³ vs. PhosphITE PO₃⁻³ (both −3). This
pattern is essential for naming oxyacids: -ate → -ic acid (H₂SO₄ = sulfuric acid), -ite → -ous acid (H₂SO₃ = sulfurous acid).
, 4. What is the formula of the nitrate ion?
A. NO₂⁻¹
B. NO₃⁻¹
C. NO₃⁻²
D. NH₄⁺
CORRECT ANSWER B — NO₃⁻¹; nitrate has three oxygen atoms and a −1 charge
RATIONALE Nitrate (NO₃⁻) has three oxygen atoms and a −1 charge. Nitrite (NO₂⁻, option A) has two oxygens and −1 charge. Both have the SAME charge (−1) —
the difference is only in the number of oxygens. Nitrate is the conjugate base of nitric acid (HNO₃, a strong acid). Option C has the wrong charge.
Option D is the ammonium cation (NH₄⁺), a polyatomic cation — not an oxyanion of nitrogen. The NO₃⁻ ion is trigonal planar with resonance-
stabilized N-O bonds.
5. What is the formula of the nitrite ion?
A. NO₃⁻¹
B. NO₂⁻¹
C. N₂O₄⁻²
D. NO⁻¹
CORRECT ANSWER B — NO₂⁻¹; nitrite has two oxygen atoms and a −1 charge
RATIONALE Nitrite (NO₂⁻) has one fewer oxygen than nitrate (NO₃⁻), following the -ate/-ite pattern. Both have −1 charge. Nitrite is the conjugate base of nitrous
acid (HNO₂, a weak acid). The nitrite ion has a bent molecular geometry with resonance between the two N-O bonds. Option A is nitrate (3 oxygens).
Option C is dinitrogen tetroxide, a neutral molecule. Option D is the hypothetical NO⁻ which is not a stable polyatomic ion.
6. What is the formula of the phosphate ion?
A. PO₃⁻³
B. PO₄⁻³
C. PO₄⁻²
D. HPO₄⁻²
CORRECT ANSWER B — PO₄⁻³; phosphate has four oxygen atoms and a −3 charge
RATIONALE Phosphate (PO₄⁻³) is a key biological and geological polyatomic ion. It has four oxygen atoms and a −3 charge. PhosphITE (PO₃⁻³, option A) has three
oxygens and the SAME −3 charge. Both follow the -ate/-ite pattern with identical charges. Option C has the wrong charge (−2 instead of −3). Option D
is the hydrogen phosphate ion (HPO₄⁻²), where one H⁺ is added to phosphate, reducing the charge by one. Phosphate is tetrahedral and is the
conjugate base of phosphoric acid (H₃PO₄).
7. What is the formula of the carbonate ion?
A. CO₂⁻²
B. CO₃⁻²
C. HCO₃⁻¹
D. CO₃⁻¹
CORRECT ANSWER B — CO₃⁻²; carbonate has three oxygen atoms and a −2 charge; it is trigonal planar with resonance
RATIONALE Carbonate (CO₃⁻²) has three oxygen atoms arranged in a trigonal planar geometry around the central carbon, with resonance delocalization of the
−2 charge. Option A (CO₂⁻²) is not a stable ion — CO₂ is carbon dioxide (neutral molecule). Option C (HCO₃⁻) is hydrogen carbonate (bicarbonate).
Option D has the wrong charge. Carbonate is found in limestone (CaCO₃), baking soda (NaHCO₃), and is the conjugate base of carbonic acid (H₂CO₃).
The carbonate ion is a classic example of resonance stabilization.
8. What is the formula of the hydroxide ion?
A. H₃O⁺
B. OH⁻¹
C. O⁻²
D. HO₂⁻¹
CORRECT ANSWER B — OH⁻¹; hydroxide has one oxygen, one hydrogen, and a −1 charge
RATIONALE Hydroxide (OH⁻) is one of the most fundamental polyatomic ions — it is the defining species of basic solutions. Arrhenius bases dissociate to
produce OH⁻ in water. Option A (H₃O⁺) is the hydronium ion — the conjugate acid of water, not hydroxide. Option C (O⁻²) is the oxide ion, a
monatomic anion. Option D (HO₂⁻) is the hydrogen peroxide anion. Hydroxide forms strong bases with Group 1 and 2 metals (NaOH, KOH, Ca(OH)₂).
9. What is the formula of the ammonium ion?
A. NH₃
B. NH₄⁺¹
C. NO₃⁻
D. NH₂⁻
CORRECT ANSWER B — NH₄⁺; ammonium is a POLYCATION (positively charged polyatomic ion) with a +1 charge
RATIONALE Ammonium (NH₄⁺) is one of the few common polyatomic CATIONS. It is formed by protonation of ammonia (NH₃ + H⁺ → NH₄⁺). It has a tetrahedral
geometry (sp3 hybridized nitrogen). Option A is ammonia (NH₃), the neutral molecule. Option C is nitrate (NO₃⁻), an anion. Option D is the amide ion
(NH₂⁻). Ammonium salts (NH₄Cl, NH₄NO₃) are common in fertilizers. The ammonium ion has a pKa of approximately 9.25, making it a weak acid.
Most other polyatomic ions are anions.
