Weak Monoprotic Acids

A weak monoprotic acid is any Bronsted-Lowry acid that possesses a single dissociable H4. The dissociation of a weak acid can be expressed in terms of the acid dissociation constant, Ka  

TABLE 1.17. Selected Dissociation Constants for Weak Acids 

The pKa values of weak Bronstead-Lowry acids generally range from 1 to 14 (Table 1.17). 

To derive a function for the buffer index of a weak monoprotic acid, HA, we begin as before with equilibrium, mass-balance, and charge-balance equations, and first derive an equation for the titration curve. We are given the following expressions  

Note that the first two bracketed terms are the buffer capacity of water, and the third term is the buffer capacity due to the weak acid. For intermediate pH s the water terms are negligible and we have 

One can now determine the pH at which a weak monoprotic acid exhibits its maximum buffer capacity (yS ,ax). This involves differentiating Eq. (5.111) with respect to pH and then setting the expression for djSIdpH equal to zero. This leads to K = [HT for the condition of maximum buffer capacity. Substituting this fact into Eq. (5.111) indicates that 

In other words, the pH for the maximum buffer capacity of a weak acid is defined by pH = ipK , and the value of is a function only of Q, the total acid-species concentration. The same reasoning applies to the maximum buffer capacity of weak bases. 

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From the point of view of quantitative analysis, sufficiently accurate values for the ionisation constants of weak monoprotic acids may be obtained by using the classical Ostwald Dilution Law expression the resulting constant is sometimes called the concentration dissociation constant . 

Boric acid behaves as a weak monoprotic acid with a dissociation constant of 6.4 x 10-10. The pH at the equivalence point in the titration of 0.2M sodium tetraborate with 0.2 M hydrochloric acid is that due to 0.1 M boric acid, i.e. 5.6. Further addition of hydrochloric acid will cause a sharp decrease of pH and any indicator covering the pH range 3.7-5.1 (and slightly beyond this) may be used suitable indicators are bromocresol green, methyl orange, bromophenol blue, and methyl red. 

Discussion. Boric acid acts as a weak monoprotic acid (Ka = 6.4 x 10"10) it... 

The weak acid curves can also be calculated. This involves the use of the equilibrium constant expression for a weak monoprotic acid ionization ... 

The answer is in the expression for the ionization constant, K l or Kb, where the ratio of the conjugate acid and base concentrations is found. In the case of a weak monoprotic acid, HA, we have the following ... 

A sample of a solid, weak monoprotic acid, HA, is supplied, along with solid sodium hydroxide, a phenolphthalein solution, and primary standard potassium hydrogen phtha-late (KHP). 

Q VSUM Para-aminohenzoic acid (PABA) is a weak monoprotic acid that is used in some sunscreen lotions. Its formula is C6H4NH2COOH. What is the formula of the conjugate base of PABA ... 

You can represent any weak monoprotic acid with the general formula HA. The equilibrium of a weak monoprotic acid in aqueous solution can be expressed as follows ... 

In low doses, harhiturates act as sedatives. Barbiturates are made from harhituric acid, a weak monoprotic acid that was first prepared by the German chemist Adolph von Baeyer in 1864. The formula... 

Caproic acid, C5H11COOH, occurs naturally in coconut and palm oil. It is a weak monoprotic acid, with = 1.3 x 10 . A certain aqueous solution of caproic acid has a pH of 2.94. How much acid was dissolved to make 100 mL of this solution ... 

All polyprotic acids, except sulfuric acid, are weak. Their second dissociation is much weaker than their first dissociation. For this reason, when calculating [HsO" ] and pH of a polyprotic acid, only the first dissociation needs to be considered. The calculation is then the same as the calculation for any weak monoprotic acid. In the Sample Problem, [HP04 ] was found to be the same as the second dissociation constant, Ka. The concentration of the anions formed in the second dissociation of a polyprotic acid is equal to... 

Iodic acid is a relatively weak monoprotic acid, the Ka value at 25°C is 1.6 x 10 h Several species have been detected in concentrated aqueous solutions, which include IO3, H+, HIO3, (HI03)2 and (HI03)3. Its solution turns blue litmus red and then bleaches the litmus paper because of its strong oxidizing properties. 

Lactic acid (C3Hg03), which occurs in sour milk and foods such as sauerkraut, is a weak monoprotic acid. The pH of a 0.10 M solution of lactic acid is 2.43. What is the value of Ka for lactic acid ... 

Because Na+ is neither an acid nor a base and CH3C02 is a weak base, we have a basic salt solution (Section 15.14), whose pH can be calculated as 8.72 by the method outlined in Worked Example 15.15. For a weak monoprotic acid-strong base titration, the pH at the equivalence point is always greater than 7 because the anion of the weak acid is a base. 

Two solutions, initially at 25.08°C, were mixed in an insulated bottle. One consisted of 400mL of a weak monoprotic acid solution of concentration 0.200 mL. The other consisted of lOOmL of a 0.800mol of NaOH per liter of solution. The temperature rose to 26.25°C. How much heat is evolved in the neutralization of one mole of the acid Assume that the densities of all solutions are 1.00 g/cm3 and that their specific heat capacities are all 4.2 J/g K. Actually, these assumptions are in error by several percent, but they nearly cancel each other. 

A student performed a titration of a weak, monoprotic acid, HA, with a sodium hydroxide, NaOH, solution. 

A 0.10-molar solution of a weak monoprotic acid, HA, has a pH of 4.00. The ionization constant of this acid is... 

A few acids contain only a single hydrogen ion that can dissociate. These acids are called monoprotic acids. (The prefix mono- means one. The root -protic refers to proton. ) Hydrochloric acid, hydrobromic acid, and hydroiodic acid are strong monoprotic acids. Hydrofluoric acid, HF, is weak monoprotic acid. 

For most neutralization reactions, there are no visible signs that a reaction is occurring. How can you determine that a neutralization reaction is taking place One way is to use an acid-base indicator. This is a substance that changes colour in acidic and basic solutions. Most acid-base indicators are weak, monoprotic acids. The undissociated weak acid is one colour. Its conjugate base is a different colour. 

Account for the fact that phosphorous acid, H3P03, is a weak, diprotic acid, whereas hypophosphorous acid, H3P02, is a weak, monoprotic acid. 

Figure 1.8. Potentiometric titration plot of a weak monoprotic acid with a pKa of 4.8 (after Segel, 1976, with permission).

In case of a weak monoprotic acid-base analyte, two different species are retained the protonated acid form (HA, uncharged or cationic) and the deprotonated conjugate basic form (A, uncharged or anionic). In other words, if HA is uncharged, A is anionic if HA is charged, A is uncharged. The ionization of the analyte is described by the following equilibrium ... 

Computation of the Ionization Constant of Weak Monoprotic Acids in Water—Organic Mixed Solvents... 

Typically, successive Ka values are so much smaller than the first value that only the first dissociation step makes a significant contribution to the equilibrium concentration of H+. This means that the calculation of the pH for a solution of a weak polyprotic acid is identical to that for a solution of a weak monoprotic acid. 

Hypophosphorous acid is a weak monoprotic acid, forming only one series of salts. The two non cidic hydrogen atoms are bonded to the phosphorus atom ... 

In calculating titration curves, separate equations for different regions of the curve ("before the equivalence point", "at the equivalence point", "after the equivalence point", etc.) are often employed. This section illustrates how to use a single "master" equation to calculate points on a titration curve. Instead of calculating pH as a function of the independent variable V, it is convenient to use pH as the independent variable and V as the dependent variable. The species distribution at a particular pH value is calculated from the [H ], and the volume of titrant required to produce that amount of each species is calculated. For example, in the titration of a weak monoprotic acid HA, we can calculate the concentration of A at a particular pH and then calculate the number of moles of base required to produce that amount of A . In general (J -j) moles of base per mole of acid are required to produce the species HjA from the original acid species HjA. 

For a weak monoprotic acidic drug, HA(soiid/iiquid), in water ... 

The dissociation of a weak monoprotic acid, HA, yields H" and A" in equal concentrations. If Ko and the initial concentration of HA are known, H" can be calculated easily ... 

Figure 1-2 Titration of a weak monoprotic acid, HA, with a strong base (e.g. KOH). pK. = 5.Q [HA] at the start = 0.1 M. ...

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