Class prep · TJ AP Chemistry
An independent AP Chemistry prep course, from atomic structure and bonding through reactions, kinetics, equilibrium, acids and bases, and electrochemistry. Connect particle models, calculations and experimental evidence. Every problem has a full worked solution. Built by Neil Kothari, a TJ alumnus.
$395 covers the full school year. The first lesson is free.
Try one. This is a test-level question from the course.
Use supplied molar masses , , and . Which pure sample contains the largest number of atoms in total?
Answer
Helium.
Full solution
The amount of constituent atoms per gram is the number of atoms per entity divided by its molar mass.
The four ratios are , , and .
Helium has the largest ratio; a molecule with more atoms can still have fewer atoms per gram.
Where students lose points
Compare atom count per mass, not the subscript alone.
Free lesson
Connect a measured sample to a clearly identified count of atoms, molecules or formula units.
Start with measured mass, moles and particle counts, then use units and complete sample definitions to check each calculation.
A pure carbon dioxide sample contains exactly of molecules. Claim: it contains of constituent atoms in total. Judge the claim; count atoms without changing the molecules.
Answer
True. Each molecule contains three atoms, so the sample contains of constituent atoms.
Full solution
The chemical subscript gives two oxygen atoms in each molecule, in addition to one carbon atom.
Multiply the molecular amount by the three atoms per molecule: of atoms.
This is a count of constituents; it does not assert that the sample is free carbon and oxygen atoms.
Where students lose points
A mole of molecules and a mole of all constituent atoms describe different entities.
Which information suffices to determine the number of argon atoms in a sample by mass? The molar mass is supplied, and is known.
Answer
A measured net mass of a sample established to be pure argon.
Full solution
Mass determines amount only after the substance and its molar mass apply to the entire measured sample.
Subtracting a container mass produces a net sample mass, which can then be divided by the argon molar mass.
An unspecified mixture, a container-inclusive mass or a volume without density leaves the atom count undetermined.
Where students lose points
The identity and purity of the measured material matter as much as the mass reading.
The full lesson and all 13 of its problems, with every worked answer.
How it works
A complete AP Chemistry course for students in grades 9 to 12. Work alongside your school course or study the material in sequence after introductory chemistry. Each lesson starts with a concrete example, develops the idea, and shows how to check your reasoning before you try the practice.
What is inside
Who built it
Neil Kothari
TJHSST alumnus · Co-founder, EduAvenues · 10+ years in TJ preparation
Neil is a TJ alumnus and has spent more than a decade working with TJ students and their families. He built this course so students can connect particle models, chemical calculations, experimental design and evidence. Every problem is written for the course and every answer is checked before it goes in.
Tuition
$395for the full school year
One tuition covers every unit of the course. All 9 units are open now. The Counting Particles Through Measured Mass lesson stays free for everyone.