Percentage deprotonation

The calculation summarized in Toolbox 10.1 also allows us to predict the percentage deprotonation, the percentage of HA molecules that are deprotonated in the solution. 

A small percentage of deprotonated molecules indicates that the acid HA is very weak. 

EXAMPLE 10.7 Sample exercise Calculating the pH and percentage deprotonation of a weak acid... 

The blue square in the grid in the final box represents the percentage of the acid molecules that are deprotonated. We see that x is less than 5% of 0.10, and the approximation is valid. Because the pH < 6, the assumption that the autoprotolysis of water can be ignored is valid. 

Sf.i f-Test 10.9A Calculate the pH and percentage deprotonation of 0.50 m aqueous lactic acid. See Table 10.1 for Ka. Be sure to check any approximation to see whether it is valid. 

In the treatment of weak acids, we found that the percentage deprotonated gave an indication of acid strength. Similarly, when we describe the strengths of weak bases, it is useful to know the percentage protonated, the percentage of base molecules that have been protonated ... 

Use Table 10.9 to determine the percentage deprotonation of 1.00 m aqueous lactic acid. At what temperature will the solution freeze (Assume that the density of the solution is 1.00 g-cm 3.)... 

Explain what happens to (a) the concentration of H,0+ ions in an acetic acid solution when solid sodium acetate is added (b) the percentage deprotonation of benzoic acid in a benzoic acid solution when hydrochloric acid is added (c) the pH of the solution when solid ammonium chloride is added to aqueous ammonia. 

The calculation of x can often be simplified, as shown in Toolbox 9.1, by neglecting changes of less than 5% ot the initial molarity of the acid. However, at the end of the calculation, we must check that x is consistent with the approximation, by calculating the percentage of acid deprotonated. If this percentage is greater than 5%, then the exact expression for X, must be solved for x. An exact calculation often involves solving a quadratic equation, as explained in Toolbox 9.1. 

Notice that percentage deprotonation is a mole percentage, not a mass percentage. 

The same calculation summarized in the toolbox also helps to make our model of acid solutions more quantitative, for it lets us predict the percentage deprotonation, the percentage of HA molecules that are deprotonated in the solution. To calculate the percentage deprotonation, we use the equality [H30+] = [A-], which follows from the stoichiometric relation 1 mol A- — 1 mol H30+ for the deprotonation reaction ... 

A small percentage of deprotonated molecules indicates that the solute consists primarily of the acid HA. The concentrations of H30+ ions and conjugate base ions must then be very low. 

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