Dissociation constant, for an acid

IQ, the acid dissociation constant, for an acid is 9 x 10-4 at room temperature. At this temperature, what is the approximate percent dissociation of the acid in a 1.0 M solution ... 

There is an important relationship between the dissociation constant for an acid, Ka, and the dissociation constant for its conjugate base, K. Consider acetic acid and its dissociation in water. 

The dissociation constant for an acid is known as the acid strength. It is a measure of the extent to which the acid produces hydrogen ions, and therefore of its strength as an acid. Bases dissociate in water as follows. 

The acid dissociation can be calculated using the same method as the dissociation of water to determine the pH. The dissociation constant for an acid is the pK and is calculated this way ... 

The above result holds for any conjugate acid-base pair. In general, the product of the acid-dissociation constant for an acid and the base-dissociation constant for its conjugate base equals the ion-product constant for water ... 

Dissociation constant For an acid, the equi-hbrium constant for the dissociation of the acid into its conjugate base and a proton. For a complex of two biomolecules, the equfiibrium constant for dissociation into the component molecules. 

Kg is the dissociation constant for an acid. It is the equilibrium constant forthe dissociation of a weak acid, HA(aq) ... 

The acid of stoichiometry H3PO3 (phosphorous acid) has a first dissociation constant typical of a moderately weak acid that is r = 1. According to Pauling s second rule, the dissociation constant for an acid with t = 0 would be expected to be around 10 mol litre which suggests that H3PO3 is not the hydroxy acid P(OH)3, but contains one P=0 group. The correct structure for this compound Q.22. 

Acid-base reactions occur when an acid donates a proton to a base. The equilibrium position of an acid-base reaction is described using either the dissociation constant for the acid, fQ, or the dissociation constant for the base, K, . The product of and Kb for an acid and its conjugate base is K (water s dissociation constant). 

The plT at which an acid-base indicator changes color is determined by its acid dissociation constant. For an indicator that is a monoprotic weak acid, ITIn, the following dissociation reaction occurs... 

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

Some compounds exhibit pH behavior in which a bell-shaped curve is obtained with maximum instability at the peak [107]. The peak corresponds to the intersection of two sigmoidal curves that are mirror images. The two inflection points imply two acid and base dissociations responsible for the reaction. For a dibasic acid (H2A) for which the monobasic species (HA-) is most reactive, the rate will rise with pH as [HA-] increases. The maximum rate occurs at pH = (pA) + pK2)/2 (the mean of the two acid dissociation constants). Where an acid and base react, the two inflections arise from the two different molecules. The hydrolysis of penicillin G catalyzed by 3,6-bis(di-methylaminomethyl)catechol [108], is a typical example. For a systematic interpretation of pH-degradation profiles, see the review papers by van der Houwen et al. [109] and Connors [110]. 

The concentration of hydrogen ions liberated by the dissociation of an acid is related to the dissociation constant for that acid and this relationship can be expressed by the Henderson-Hasselbalch equation ... 

The solvent isotope effect on the dissociation constant of an acidic proton in aqueous solution has been used to deduce a value for the fractionation... 

Hasselbalch equation, which is important for understanding buffer action and acid-base balance in the blood and tissues of vertebrates. This equation is simply a useful way of restating the expression for the dissociation constant of an acid. For the dissociation of a weak acid HA into H+ and A-, the Henderson-Hasselbalch equation can be derived as follows ... 

The DSP approach nicely answers the controversial question about which substituent parameters should be employed to correlate pKa data for 4-substituted pyridinium ions. Statistically, the best correlation is given by Eq. (9), which has values to measure the resonance contribution of a substituent, a result in keeping with chemical intuition. This correlation is statistically superior to a Hammett treatment, where both resonance and inductive effects of a group are combined into a single parameter, p or ap.53,54 Moreover, now it is possible to rationalize why a simple Hammett treatment using ap works so well. Equation (9) reveals that the protonation equilibrium is much more sensitive to an inductive effect (p, — 5.15) than to a resonance effect (p = 2.69). Hence, substituent parameters, such as erp, which are derived from a consideration of the dissociation constants for benzoic acids where resonance contributions are small serve as a useful approximation. The inductive effect is said to have a larger influence on pKa values for pyridinium ions than for benzoic acids because the distance between the substituent and the reactive site is shorter in the pyridine series.53... 

Fast atom bombardment mass spectrometry has been utilized for the quantitative determination of ionic species, in glycerol/water solutions, which are produced by chemical and enzymic reactions. It is shown that reaction constants can be determined in this manner and that they can be accurately related to those determined by other methods used in the analysis of aqueous solutions. The reactions studied include proton dissociation constants for organic acids, an enzyme equilibrium constant, and enzyme rate constants using natural substrates. 

Cyclohexanol is a typical secondary alcohol, with a typical acid-dissociation constant for an alcohol. There must be something special about phenol that makes it unusually acidic. The phenoxide ion is more stable than a typical alkoxide ion because the negative charge is not confined to the oxygen atom but is delocalized over the oxygen and three carbon atoms of the ring. 

The equilibrium constant of dissolution of an electrolyte (describing the equilibrium between excess solid phase and solvated ions) is often called a solubility product, denoted Ksol or Ks (or KSoi or K as appropriate). In a similar way the equilibrium constant for an acid dissociation is often written Ka, for base hydrolysis Kb, and for water dissociation Kw. 

In solvents of moderately high dielectric constant, the dissociation constant of an acid is determined primarily by the basicity of the solvent. Thus in two solvents S H and S"H with similar dielectric constants, for a particular acid HA we may write the equilibria... 

An acid will have its own equilibrium constant in water, called the acid dissociation constant Kn. If we use our hypothetical acid-base reaction HA + H20 HsO + A", then the acid dissociation constant for the acid HA is ... 

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