A chemical formula connects molecules to atoms
You will be able to: Use formula subscripts to calculate molar mass and atom counts.
How many oxygen atoms are in a sample of CO₂?
One CO₂ molecule contains one carbon atom and two oxygen atoms. Two molecules therefore contain four oxygen atoms, and half a mole of molecules contains one mole of oxygen atoms.
A useful starting point: Use mass to count an enormous number of particles →
Words and symbols before equations
- Subscript
- Small number in a formula: O₂ means two O atoms per molecule.
- Molecule
- A discrete group of bonded atoms.
- Formula unit
- The simplest ratio represented by an ionic formula; it is not necessarily a separate molecule.
What this picture assumes
Pure CO₂; rounded C = 12 and O = 16 g/mol. Counts refer to molecules and atoms within them, not free C or O₂. Bars show mole amounts on a common scale.
Read the picture in three steps
- Identify the chemical species and the quantities each label or axis represents. Read the units and any scale assumptions before comparing values.
- Mass = 22 g; molecules = 3.011 × 10²³; O atoms = 6.022 × 10²³; total atoms = 9.033 × 10²³.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Add the atomic-mass contributions to obtain molar mass. With C = 12.0 and O = 16.0 g/mol, M(CO₂) = 12.0 + 2(16.0) = 44.0 g/mol.
First find moles of molecules from the whole substance mass. Only then use the formula ratio: 2 mol of O atoms for each 1 mol of CO₂ molecules.
CO₂ has three atoms per molecule in total. An O atom within CO₂ is not a free O₂ molecule. Ionic solids such as NaCl are counted in formula units because they form extended arrangements.
A worked example, step by step
Find the moles of CO₂ molecules and O atoms in 11.0 g of CO₂, using M = 44.0 g/mol.
- Start with the whole compound: 11.0/44.0 = 0.250 mol CO₂.
- Use the subscript ratio: 0.250 × 2 = 0.500 mol O atoms.
- Multiply by Nₐ: 0.500 × 6.022 × 10²³ = 3.01 × 10²³ O atoms.
- The same sample contains 0.750 mol total atoms, because each molecule has three.
Do not interpret the oxygen atoms in CO₂ as O₂ molecules.
Does 1 mol CO₂ contain 1 mol total atoms?
Compare with an explanation
No. It contains 1 mol C atoms and 2 mol O atoms: 3 mol total atoms.
Predict. Change one thing. Explain.
Increase the amount of CO₂. Compare molecule count, oxygen-atom count and total-atom count on the same scale.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Mass = 22 g; molecules = 3.011 × 10²³; O atoms = 6.022 × 10²³; total atoms = 9.033 × 10²³.
Pure CO₂; rounded C = 12 and O = 16 g/mol. Counts refer to molecules and atoms within them, not free C or O₂. Bars show mole amounts on a common scale.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use particle counts, mass or charge balance, electron structure, or nuclear attraction to justify your prediction. Separate an observation from an explanation.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionUsing Nₐ = 6.022 × 10²³, analyze 0.300 mol NH₃: molecules, moles of H atoms, total moles of atoms, and why the counted species matter.
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Compare with the answer and four-point rubric
- 1 point: Molecules: 0.300Nₐ = 1.81 × 10²³.
- 1 point: H atoms: 3(0.300) = 0.900 mol.
- 1 point: Total atoms: 4(0.300) = 1.20 mol.
- 1 point: NH₃ molecules, individual H atoms and H₂ molecules are distinct entities; no free H₂ is implied.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What multiplies the molecule amount to give O atoms in CO₂?
The subscript 2.
RECALL 2How do you calculate molar mass from a formula?
Sum each atomic molar mass times its subscript.
RECALL 3Why use formula units for ionic solids?
Their formulas describe ratios in extended structures.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
A chemical formula connects molecules to atoms
- M(CO₂) = M(C) + 2M(O).
- n(O atoms) = 2n(CO₂).
- N(total atoms) = 3n(CO₂)Nₐ.
Remember: Do not interpret the oxygen atoms in CO₂ as O₂ molecules.
Conditions: Pure CO₂; rounded C = 12 and O = 16 g/mol. Counts refer to molecules and atoms within them, not free C or O₂. Bars show mole amounts on a common scale.
Refresh Kid · AP Chemistry Unit 1 · Objectives 1.1.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 1.1, objectives 1.1.A. CED effective Fall 2024, current official file checked September 16, 2026, together with the published clarifications. This is Unit 1: Atomic Structure and Properties, Topics 1.1–1.8. The topic mapping identifies a framework area; focused lesson titles are our own teaching sequence. Molecular-formula scaling is an application of empirical composition. Models explicitly distinguish atom counts, molecule counts, mass fractions and electron structure. Spectra marked schematic are not measured data. Mass spectra here use single-element, singly charged monatomic ions. Configurations avoid Aufbau exceptions and individual quantum-number assignments. Qualitative attraction and size indices are not exact atomic predictions. The optional NaCl-type spatial block supplements complete charge-balance explanations. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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