Take, for example the three dissociation steps of the common triprotic acid phosphoric acid: [latex]\text{H}_3\text{PO}_4(\text{aq})\rightarrow \text{H}^+(\text{aq})+\text{H}_2\text{PO}_4^-(\text{aq})\quad\quad \text{K}_{\text{a}1}=\text{large}[/latex], [latex]\text{H}_2\text{PO}_4^-(\text{aq})\rightleftharpoons \text{H}^+(\text{aq})+\text{HPO}_4^{2-}(\text{aq})\quad\quad \text{K}_{\text{a}2}=\text{small}[/latex], [latex]\text{HPO}_4^{2-}\rightleftharpoons \text{H}^+(\text{aq})+\text{PO}_4^{3-}(\text{aq})\quad\quad \text{K}_{\text{a}3}=\text{smallest}[/latex]. Moreover, with complete AV block, the ventricular rate is almost always slower than the atrial rate, whereas in other forms of AV dissociation, the reverse is true.11,35 Therefore, complete AV block with a junctional or ventricular escape rhythm is one form of AV dissociation. People who experience a traumatic event will often have some degree of dissociation during the event itself or in the following hours, days or weeks. The salt is 60% dissociated. With this type of AV dissociation, very distinct from complete heart block, the SA node and AV junction appear to be “out of synch”; thus, the SA node loses its normal control of the ventricular rate. AV dissociation is also used as a more specific term to describe a particular family of arrhythmias that are often mistaken for complete heart block. The larger the K value, the more the reaction will tend toward the right and thus to completion. Chemical Principles/Solution Equilibria: Acids and Bases. K a is a better measure of the strength of an acid than pH because adding water to an acid solution doesn't change its acid equilibrium constant, but does alter the H + ion concentration and pH. Following Wikipedia's van 't Hoff factor discussion, the van 't Hoff factor can be computed from the degree of ionization as follows: where α is the degree of dissociation and n equals the number of ions formed from one formula unit of the substance. The dominant rhythm changes as one slightly slows or speeds its rate. AV dissociation is widely used as a general term for any arrhythmia in which the atria and ventricles are controlled by independent pacemakers. [Ba2+] = 2 M times (1 x 0.15) = 0.3 M Three clues may be especially helpful in favoring VT. AV dissociation is an important clue that may be helpful in differentiating SVT with aberrancy from VT. Recall from Chapter 17 that, with AV dissociation, the atria and ventricles are paced from separate sites. We can simplify the problem, depending on the polyprotic acid. The reasoning to get the van 't Hoff factor is broken down in a little bit different way than what I did above in previous problems. The critical difference between AV dissociation resulting from “desynchronization” of the SA and AV nodes from that of conduction failure and complete heart block is as follows: with AV dissociation (e.g., isorhythmic type) a properly timed P wave can be conducted through to the AV node, whereas with complete heart block, no P wave can stimulate the ventricles. If it does not look like either one, the rhythm is usually VT. A P wave before each wide QRS suggests SVT with aberrancy. A salt is considered soluble if it dissolves in water to give a solution with a concentration of at least 0.1 M at room temperature. 20-13). The greater the value of K a, the more favored the H + formation, which makes the solution more acidic; therefore, a high K a value indicates a lower pH for a solution. Some patients with VT also have AV dissociation; in other words, the ventricles are stimulated from an ectopic site at a rapid rate, while the atria continue to be paced independently by the SA node. When a weak diprotic acid such as carbonic acid, H2CO3, dissociates, most of the protons present come from the first dissociation step: [latex]\text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-[/latex]  pKa1 = 6.37. Example #1: What is the expected van 't Hoff factor for a substance (such as glucose) that does not ionize at all in solution. Polyprotic acids contain multiple acidic protons that can sequentially dissociate from the compound with unique acid dissociation constants for each proton. Diprotic and polyprotic acids contain multiple acidic protons that dissociate in distinct, sequential steps. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. 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