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CHEM 102: The Acid-Base Survival Blueprint Master the concepts, dodge the exam traps, and crush your quiz in 10 minutes. Ol Red = Acid (Proton Donor) | Blue = Base (Proton Acceptor) CT] Gray = Neutral / Spectator A\ NotebookLM THE PLAYERS: DONORS AND ACCEPTORS Brgnsted-Lowry Acids donate protons (H+). Bases accept them. A conjugate pair is simply the before-and- after versions of the same molecule, differing by exactly one H+ and one unit of charge. Conjugate Pair 1 PS HA + B == A+ HBS Conjugate Pair 2 QUIZ EXAMPLE Q: Conjugate base of H,SO,? -> HSO, Q: Conjugate acid of CH,CH,NH,? -> CH,CH,NH,* EXAM TRAP: Forgetting to adjust the the charge goes up by 1. QUICK TRICK: Acid to Conjugate Base: Subtract H, Subtract 1. charge! If you remove an H+, the charge drops by 1. If you add an H+, ? Base to Conjugate Acid: Add H, Add 1 A\ NotebookLM THE SEESAW: STRENGTH AND PKA The stronger the acid, the weaker its conjugate base. Because pKa is a negative logarithm, a lower pKa means a stronger acid. Can mix kpa , kpb so convert to one QUIZ EXAMPLE Beaker A: Boric Acid (pKa = 9.23) Beaker B: Acetic Acid (pKa = 4.75) Beaker C: H3P04 (pKa = 7.21) Q: Highest [H30+]? -> Acetic (lowest pKa). Q: Strongest conjugate base? -> Boric (highest pKa). A\ NotebookLM THE SALT SORTING HAT HYDROLYSIS MATRIX Salts dissolve into cations (+) and anions (-). i Strong Cation + Strong Anion = NEUTRAL (pH = 7) . ; Example: CsC103 To find the pH of the salt solution, cross out the “spectators” (conjugates of strong 2 Weak Base Cation + Strong Anion = ACIDIC (pH < 7) acids/bases) . Example: NH4Br Whatever survives dictates the pH. 3 Strong Cation + Weak Acid Anion = BASIC (pH > 7) Example: NaHS QUIZ EXAMPLE EXAM TRAP: Thinking all salts are automatically neutral. Q: Which salt produces a pH closest to 7.00? A) NH4Br (Acidic) QUICK TRICK: Cross out the Weaklings. B) Ca(HC03)2 (Basic) Instantly cross out Group 1/2 metals (Nat, C) H3P04 (Acid) Ca2+, Cs+) and strong acid anions (Cl-, D) CsC103 Br-, NO3-, C104-). If itive ion is left, it’ idie. Lf Answer: D. Both Cst+ and C103- are strong epaglives mt grit 5 2 Se ae spectators. A\ NotebookLM THE pH SQUARE & THE MAGIC 14 [H+] < > [OH-] Water's autoionization links H+ and OH- perfectly. At 25°C, pH and pOH will always " add up to exactly 14. pH + pOH = 14 You can navigate between any of the four corners using simple formulas. pH ==> pOH QUIZ EXAMPLE Q: A solution of Ba(0H)2 has a pH of 12. Concentration? ‘ EXAM TRAP: Forgetting stoichiometry for Group 2 bases! Ba(OH)2 produces TWO moles of OH- for every mole of base. QUICK TRICK: Never calculate [H+] to _ ) find the concentration of a base. It takes too long. Always switch to p0OH first using 14 - pH. Math: pH = 12 > pOH = 2 > [OH-] = 104-2 = Q.01M. Because Ba(0H)2 gives TWO OH- per molecule: a || Concentration = 8.01 / 2 = @.005M. y, A\ NotebookLM The Math Bridge for Conjugates For any exact conjugate acid-base pair, their equilibrium constants are mathematically locked together by the constant of water (K, = 1.0 x 104-14). Ka X Ky = 1.0 x 10*-14\\) Check at first is is acid in water or base in water QUIZ EXAMPLE Q: You have an unknown acid HB with a pOH of 10. The pKb for its conjugate base B- is 5.00. Math: To find properties of the acid HB, you CANNOT use pKb. You must cross the bridge: pKa = 14 - 5.00 = 9.00. EXAM TRAP: Trying to use Kg X Kp = K, for two completely unrelated molecules. It ONLY works for a conjugate pair! QUICK TRICK: If the problem gives you a base but asks about an acidic salt (or vice versa), your very first step is to use the “14" rule to flip the constant. A\ NotebookLM Escaping “p-Land” | ------- ' Ka = 19-Pka ICE tables run on Molarity and K , p-Land i constants. You cannot plug a pH, | pKa | Molarity / pKa, or pKkb directly into an ! K-Land ie , 1 pKb .f equilibrium expression. You must Moo ese unk ' Kb = 109-PKb convert them out of “p-Land” first. QUIZ EXAMPLE EXAM TRAP: Accidentally writing 9.22 = Q: The pKb of a base is 4.78. We (x42) / 1.5 - in the ICE formula. You Se will get an entirely wrong, need the Ka for the ICE table. mathematically impossible answer. Step 1: pKa = 14 - 4.78 = 9.22 QUICK TRICK: ‘p’ literally just means e ae ‘-log'. To undo a ‘p’, make the number a : - 19°-9.22 - 10 Step 2: Ka = 16 = 6.0 x 16 negative exponent over a base of 10. Do this before setting up your ICE table! A\ NotebookLM 1 | Step 1: Identify the speci The Boss Battle: Setup elihageie pe 1 Gloag Cross out Br~ (neutral Quiz Q5: If the pKb of (CH3) NH (CH) NHB spectator). We are left is 4.78, what would be the pOH CA IE Ace of a 1.5M (CH3)oNH2Br solution? rea Step 2: Get the right constant. We have pK, (4.78), but we need a K, for our acid. Let’s break the setup down. pKa = 14 - 4.78 = 9.22 Ka = 10°-?: = 6.0 x 10°” acid! Ignore: Sigel A al EXAM TRAP: Treating (CH3)9NH2Br as a base because it has Nitrogen in it. 3 y) The positive charge makes it an QUICK TRICK: Halogen salts ending in (CH, ) >NH,* Active ¥ Cl, Br, I are almost always acidic : problems in disguise. Drop the Weak Acid Halogen, treat as a weak acid. A\ NotebookLM The Exam Day Mind Map How to attack any Acid-Base problem on the test. Follow the logic flow to know exactly which formulas to use. Decision 1: Is it | Strong or Weak? a Strong -> 100% dissociation. Weak -> Go to No ICE table. Use Decision 2. Stoichiometry directly. J Decision 2: Acid, Base, or Salt? Acid (Red Box) -> Base (Blue Box) -> Need Ka. Run ICE Need Kb. Run ICE table. Solves table. Solves for [H+]. for [OH-]. Salt (Gray Box) -> Cross out strong spectators. Find if the remainder is conjugate to a weak acid/base. Flip Ka/Kb using 14. Run ICE. unfamiliar. exam. MI EXAM TRAP: Panicking when a problem looks perfectly to one of these paths. Find your QUICK TRICK: Every problem on the test maps ® path, trust the blueprint, and crush the A\ NotebookLM 
