Percent Composition, Empirical & Molecular Formulas — Practice · Grade 11 Science

Chapter: Reactions, Moles & Composition

What this practice covers

By the end of Percent Composition, Empirical & Molecular Formulas you should be able to:

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

Vocabulary used: empirical formula, molecular formula, percent composition.

Problem 1

With C=12 and O=16, find percent carbon in CO₂.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

27.27 %

12/44 ×100 = 27.27%.

Problem 2

Why not use mass ratios directly as formula subscripts?
Answer and explanation

Subscripts count atoms; convert mass to moles because elements have different atomic masses.

Response criteria

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

Problem 3

An empirical formula mass is 15 g/mol and the molecular molar mass is 60 g/mol. Find the subscript multiplier.
Hints

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

Answer and explanation

4

60/15=4.

Problem 4

Empirical mass is 46 g/mol and molecular mass 92. Find multiplier.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

2

92/46 = 2.

Problem 5

An empirical formula is CH₂O, mass 30 g/mol; measured molar mass is 90 g/mol. Find molecular formula. Show the reasoning, not just the conclusion.
Hints

Percentages describe mass, so convert to moles before atom ratios.

Answer and explanation

90/30=3; multiply subscripts to get C₃H₆O₃.

Problem 6

A compound has molar masses 24 g of C and 4 g of H in the analyzed portion. Use C=12 and H=1 to find empirical formula.
Hints

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

Answer and explanation

Moles C=2, H=4; ratio 1:2, giving CH₂.

Problem 7

Near-simple fractions should be tested against the evidenceNear-simple fractions should be tested against the evidenceInitial ratio: 1.00 : 1.50Multiply every term by 2Whole-number ratio: 2 : 3Do not round 1.50 to 2 before checking a simple multiplier.
Original fractional-ratio demonstration without an assigned chemical species or measurement uncertainty.
Interpret the supplied visual. Why is multiplying the entire 1:1.5 ratio by two preferable to rounding only 1.5?
Answer and explanation

It preserves the measured proportional relationship, yielding 2:3; rounding one term changes the ratio.

An empirical ratio near 1:1.5 can support a 2:3 atom ratio after multiplying all terms by two. Premature rounding to 1:2 changes the composition substantially. Look for simple fractional relationships and compare the reconstructed percent composition with the original data and uncertainty. A complicated multiplier chosen only to force an answer can overfit noisy measurements. If several formulas fit within error, report that ambiguity and request more evidence rather than treating the nearest decimal as an exact atomic count.

It preserves the measured proportional relationship, yielding 2:3; rounding one term changes the ratio.

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

A selected oxide sample has 70.0 g Fe and 30.0 g O. Use Fe=56.0 and O=16.0 g/mol to find the simplest formula. State one assumption.
Answer and explanation

Amounts are 1.25 mol Fe and 1.875 mol O, ratio 1:1.5, hence Fe₂O₃. Assume the analyzed sample is one oxide accounted for by those elements and the input precision supports that ratio.

Amounts are 1.25 mol Fe and 1.875 mol O, ratio 1:1.5, hence Fe₂O₃. Assume the analyzed sample is one oxide accounted for by those elements and the input precision supports that ratio.

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