6b.3: Solving
Two Critical Questions:
1. Is my compound an acid or a base?
2. Is it strong or weak?
1. Is my compound an acid or a base?
2. Is it strong or weak?
Strong Acid:
Because it dissociates completely, [H+] = [acid], so pH = -log[acid] Strong Base: Same principle as acid, but with pOH: pOH = -log[base] and pH + pOH = 14 EXCEPT!!!! * Oxide makes two hydroxides when dissolved in water--it makes one and becomes one. * Some compounds have multiple hydroxides--such as calcium hydroxide In both of these cases, the [OH-] is double the [base]. (In principle, you'd triple it for aluminum hydroxide...if you can somehow get it to dissolve) |
Weak Acid:
Write a Ka and do an ICE box to get [H+]. Then pH = -log[H+] Weak Base: Write a Kb and do an ICE box to get [OH-]. Then pOH = -log[OH-] and pH + pOH = 14 However, you will rarely be given Kb. Instead, you need to use the equation: Ka*Kb = Kw Where Ka and Kb are for a conjugate acid-base pair (so you'd look up the Ka of HF if you wanted to get the Kb of F-) |