Chemical Names, Formulas & Polyatomic Ions — Practice · Grade 11 Science

Chapter: Bonding, Formulas & Molecular Structure

What this practice covers

By the end of Chemical Names, Formulas & Polyatomic Ions you should be able to:

8 questions, written answer and numeric, each with a worked explanation.

Vocabulary used: polyatomic ion, Roman numeral, subscript.

Problem 1

Write the formula of magnesium nitrate.
Answer and explanation

Mg(NO₃)₂: one Mg²⁺ balances two NO₃⁻.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 2

Count oxygen atoms in 2Al₂(SO₄)₃.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

24 atoms

2 ×3 ×4 = 24.

Problem 3

Count oxygen atoms represented by 4Ca(NO₃)₂.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

24 atoms

4×2×3=24.

Problem 4

Why is calcium carbonate CaCO₃ rather than Ca₂(CO₃)₂?
Answer and explanation

Ca²⁺ and CO₃²⁻ balance in the smallest 1:1 ratio.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 5

Write the formula for calcium nitrate using Ca²⁺ and NO₃⁻. Show the reasoning, not just the conclusion.
Hints

The Roman numeral gives oxidation state, not number of iron atoms.

Answer and explanation

Ca(NO₃)₂; two nitrate ions balance calcium's +2 charge.

Problem 6

Name CuCl₂ assuming chloride is −1.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Copper(II) chloride, since two chlorides require Cu²⁺.

Problem 7

Keep a polyatomic ion intact while balancing its formula ratioKeep a polyatomic ion intact while balancing its formula ratioCa²⁺ + 2 NO₃⁻ → Ca(NO₃)₂One nitrate: 1 N + 3 OTwo nitrates: 2 N + 6 OParentheses multiply the entire polyatomic group.Nitrate NO₃⁻ and nitrite NO₂⁻ are different ions.
Original stoichiometric formula construction; the reaction-style arrow indicates combining counts, not a complete solution reaction.
Interpret the supplied visual. How many oxygen atoms are represented by Ca(NO₃)₂, and why is CaNO₆ not an equivalent formula account?
Answer and explanation

Six oxygen atoms occur in two nitrate groups, along with two nitrogen atoms. CaNO₆ would lose a nitrogen and fail to represent the stated ions.

A polyatomic ion is a charged group of covalently connected atoms, such as nitrate NO₃⁻ or sulfate SO₄²⁻. Its internal formula is retained when charge balance determines how many groups are needed. Calcium nitrate therefore needs two nitrate ions, written Ca(NO₃)₂, giving one Ca, two N and six O atoms. Parentheses group the ion; they do not change its internal subscript to force neutrality. Learn common ion names and charges as linked information, and distinguish similarly named but chemically different ions.

Six oxygen atoms occur in two nitrate groups, along with two nitrogen atoms. CaNO₆ would lose a nitrogen and fail to represent the stated ions.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 8

Write the formula for iron(III) sulfate and count all atoms per formula unit. Then name N₂O₃ using molecular prefixes.
Answer and explanation

Fe³⁺ and SO₄²⁻ require Fe₂(SO₄)₃: two Fe, three S and twelve O atoms. N₂O₃ is dinitrogen trioxide. The Roman numeral and prefixes serve different bookkeeping roles.

Fe³⁺ and SO₄²⁻ require Fe₂(SO₄)₃: two Fe, three S and twelve O atoms. N₂O₃ is dinitrogen trioxide. The Roman numeral and prefixes serve different bookkeeping roles.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Browse all courses