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Calculating the Equilibrium Constant

An ICE tableInitial, Change, Equilibrium — organizes a K calculation. Fill in the initial concentrations, express the changes in the coefficient ratio (using a variable), and add to get the equilibrium row. Then compute K from the equilibrium concentrations.

UNIT 7 TOPIC 7.4 • CALCULATING THE EQUILIBRIUM CONSTANT ICE TABLE BUILDER EXAMPLE: Kc FROM CONCENTRATIONS N₂O₄(g) ⇌ 2 NO₂(g) [N₂O₄] [NO₂] Initial 0.100 0 Change −x +2x Equil. 0.100 − x 2x Kc = [NO₂]² / [N₂O₄] Kp FOR GASES Kp uses partial pressures Kp = (PNO₂)² / PN₂O₄ Use Kc for molarity and Kp for gas partial pressures. Do not mix units inside one expression. RULES WHILE BUILDING ICE Coefficients become exponents in K. Coefficients also scale changes in the ICE row. Pure solids and liquids are omitted. NO COLUMN COEFFICIENTS Column header: [NO₂] NOT: [2 NO₂] The coefficient belongs in the balanced equation and in the change row. It tracks the concentration of one species. CED ANCHOR An ICE table tracks initial, change, and equilibrium amounts. K expressions use equilibrium amounts only, with coefficients as exponents. AP Chemistry · Unit 7 · Equilibrium
An ICE table tracks Initial, Change, and Equilibrium concentrations. The changes follow the stoichiometry (in the coefficient ratio); the equilibrium row is initial plus change. K is calculated from the equilibrium row — never the initial row.
ICE Table Builder · Open the sandbox →

The traps read K off the initial row (K uses equilibrium values), mishandle the change row (the changes follow the stoichiometry and have signs), or treat the variable x as the final answer rather than substituting it back. Build the equilibrium row, then calculate K.

The work

3 ways in · any order
Lesson
Calculating K

An ICE table builds the equilibrium row from initial plus change (in the coefficient ratio), and K comes from that row. The lesson works the table, then closes with a ten-scenario check.

Skill check · 10 scenarios
Diagnostic
10-item topic check

Ten items spanning the Topic 7.4 misconceptions: quotient built wrong, K read off the initial row, the ICE change row mishandled, and x misread as the final answer.

Not started · 10 items · ~15 min
Targeted Practice
Drill a single misconception

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions