valence

What Valence does

One place to prepare for the U.S. National Chemistry Olympiad. Free, no account needed, and your progress stays on this device until you sign in.

  1. 01

    Every past exam

    1,354 questions from USNCO local and national Part I, 2016–2026, with the 161 figures, graphs and structures they depend on. Every question is tagged to the official ten topic blocks and attributed to its exam and number.

  2. 02

    Part II, graded

    96 free-response problems from national Part II. Type an answer per sub-part and get points against the official key and a rubric, with what was missing and the mistake you likely made. Nobody else grades Part II.

  3. 03

    Practice that adapts

    Each question is chosen from what you have missed, what you have not seen, and how hard the field found it. A per-topic rating updates after every answer, so the weak spots surface on their own.

  4. 04

    An explanation for everything

    1,354 explanations: a worked path to the answer and a line on why each wrong option is wrong. Drafted by a model, marked as drafts until a verifier signs off.

  5. 05

    Real mock exams

    60 questions in the real format and topic order, on the real clock: 110 minutes for local, 90 for national. The timer survives a refresh. Your score is broken down by topic and the misses go to Review.

  6. 06

    A predicted score

    After 40 answers, an estimate of your local or national score, labelled rough until there is enough data. Missed questions come back on a spaced schedule so they stay learned.

Start

PracticeTen questions takes about three minutesPart IIPick a problem, type an answer, get gradedMock examThe full 60, timedWhat’s on the examThe ten topic blocks, ranked by how hard qualifiers found them

Questions are © American Chemical Society, reproduced for non-commercial practice; Valence is not affiliated with ACS. If you are a USNCO alum and want to verify explanations, get in touch.