Equivalent Mass of an Acid

Preparation Sign in at www.cengage.com/login to read the MSDSs for the chemi-cais used or produced in this procedure. 

Accurateiy weigh about 0.2 g of the acid and dissoive it in 50-100 mL of water or 95% ethanoi or a mixture of the two. It may be necessary to warm the mixture to dissoive the compound compieteiy. Using phenoiphthaiein as the indicator, titrate the soiution with a sfanc/ard/zed sodium hydroxide soiution having a concentration of about 0.1 M (Fig. 25.6). From the data obtained, caicuiate the equivaient mass. 

Three satisfactory solid derivatives of carboxylic acids are amides (Eq. 25.46), anilides (Eq. 25.47), and p-toluidides (Eq. 25.48). These derivatives are prepared by treating the corresponding acid chlorides with ammonia, aniline, or p-tolui-dine. The amides are generally less satisfactory derivatives than the other two because they tend to be more soluble in water and thus are harder to isolate. The acid chlorides are most conveniently prepared from the acid, or its salt, and thionyl chloride (Eq. 25.49). 

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The equivalent mass of an acid is that fraction of the molar mass which contains or can supply one mole of H+. A simple way of looking at the equivalent mass is that it is the mass of the acid divided by the number of H s per molecule, assuming that complete ionization occurs. 

What is the equivalent mass of an acid 1.243 g of which required 31.72mL of 0.192 3N standard base for neutralization ... 

It is possible to carry out the hydrolysis of an ester with alkali in a quantitative manner so that the saponification equivalent, SE, results. This value is analogous to the equivalent mass of an acid (Sec. 25.13) in that it is the molar mass of the ester divided by the number of ester functions in the molecule. Therefore the SE is the number of grams of ester required to react with one gram-equivalent of alkali. 

One equivalent mass of an acid neutralizes one equivalent mass of a base. It is important to note that the equivalent mass of a polyprotic acid is not invariant for H3PO4 it may be the gram formula mass, one half this, or one third, depending on whether one, two, or three hydrogens are effective in the reaction under consideration. 

In ordinary acid-base reactions there are one, two, or three equivalents per mole. It follows that the equivalent mass of an acid or a base is the same as, one half of, or one third of the molar mass. The molar mass of phosphoric acid is 97.99 g/mol. For the three reactions of phosphoric acid (Equations 16.6, 16.7, and 16.8), the equivalent masses are... 

In an experiment to determine the equivalent mass of an unknown acid, a student measured out a 0.250-gram sample of an unknown solid acid and then used 45.77 mL of 0.150 M NaOH solution for neutralization to a phenolphthalein end point. Phenolphthalein is colorless in acid solutions but becomes pink when the pH of the solution reaches 9 or higher. During the course of the experiment, a back-titration was further required using 1.50 mL of 0.010 M HC1. 

Once the student had determined the exact concentration of the base, the student then proceeded to determine the equivalent mass of an unknown acid. To do this, the student measured out 0.500 grams of an unknown solid acid and titrated it with the standardized base, recording pH with a calibrated pH meter as the base was added. The student added 43.2 mL of the base but went too far past the end point and needed to back-titrate with 5.2 mL of the 0.100 M HC1 to exactly reach the end point. 

In a neutralization reaction, moles of H+ ions equal moles of OH ions. This relationship is the basis for the procedure called titration, which you will use to standardize a base solution. Standardizing a base means determining its molar concentration. You will then use your standardized base to determine the molar mass of an acid. To determine when the moles of H+ equal the moles of OH, you will monitor the pH of an acid solution as a solution of base is added slowly. The pH will rise suddenly when the concentrations of the two ions are equal (the equivalence point). 

EXAMPLE 15.22. Calculate the molar mass of an acid with three replaceable hydrogen ions and an equivalent mass of 31.2 g/equiv, assuming complete neutralization. 

The equivalent weight of an acid or base is the mass of the acid or base in grams that reacts with or contains one mole of protons. Thus, the equivalent weight of KOH (56.11 g/mol) is its molar mass for Ba(OH)2, it is the moiar mass... 

Equivalent weight in acid-base reactions The mass of an acid or base that furnishes or reacts with 6.022 X 10 H3O+ or OH ions. 

Normality is another unit of concentration that is sometimes used, especially when dealing with acids and bases. The use of normality focuses mainly on the H and OH" available in an acid-base reaction. Before we discuss normality, however, we need to define some terms. One equivalent of an acid is the amount of that acid that can furnish 1 mol ofH ions. Similarly, one equivalent of a base is defined as the amount of that base that can furnish 1 mol ofOH ions. The equivalent weight of an acid or a base is the mass in grams of 1 equivalent (equiv) of that acid or base. 

A 0.0809-g sample of a purilied organic acid was dissolved in an alcohol-water mixture and titrated with cttulometrically generated hydroxide ions. With a current of 0.0441 A. 266 s was required to reach a phenolphthalein end point. Calculate the equivalent mass of the acid. 

Another concentration measure sometimes encountered is normality (symbolized by N). Normality is defined as the number ot equivalents per liter of solution, where the definition of an equivalent depends on the reaction taking place in the solution. For an acid-base reaction, the equivalent is the mass of acid or base that can furnish or accept exactly 1 mole of protons (H ions). In Table 11.2 note, for example, that the equivalent mass of sulfuric acid is the molar mass divided by 2, since each mole of H2SO4 can furnish 2 moles of protons. The equivalent mass of calcium hydroxide is also half the molar mass, since each mole of Ca(OH)2 contains 2 moles of OH ions that can react with 2 moles of protons. The equivalent is defined so that 1 equivalent of acid will react with exactly 1 equivalent of base. 

What is one equivalent of an acid What does an equivalent of a base represent How is the equivalent weight of an acid or a base related to the substance s molar mass Give an example of an acid and a base that have equivalent weights equal to their molar masses. Give an example of an acid and a base that have equivalent weights that are not equal to their molar masses. What is a normal solution of an acid or a base How is the normality of an acid or a base solution related to its molarity Give an example of a solution whose normality is equal to its molarity, and an example of a solution whose normality is not the same as its molarity. 

The acidity of carboxylic acids enables ready determination of the equivalent mass or neutralization equivalent of the acid by titration with standard base. The equivalent weight of an acid is that mass, in grams, of acid that reacts with one equivalent of base. As an example, suppose that 0.1000 g of an unknown acid requires 16.90 mL of 0.1000 N sodium hydroxide solution to be titrated to a phenolph-thalein endpoint. This means that 0.1000 g of the acid corresponds to (16.90 mL) (0.1000 equivalent/1000 mL) or 0.0016901 equivalent of the acid, or that one equivalent of the acid weighs 0.1000/0.00169 or 59.201 g. Thus the following expression applies ... 

The quotient of the gram formula mass of an acid by the number of hydrogen atoms which are replaceable for the reaction under consideration is called the equivalent mass of the acid. Likewise, the quotient of the gram formula mass of a base by the number of hydroxyl groups which are replaceable for the reaction under consideration is called the equivalent mass of the base. 

Given an equation for a neutralization reaction, state the number of equivalents of acid or base per mole and calculate the equivalent mass of the acid or base. 

Neutralisation reactions. The equivalent of an acid is that mass of it which contains 1.008 (more accurately 1.0078) g of replaceable hydrogen. The equivalent of a monoprotic acid, such as hydrochloric, hydrobromic, hydriodic, nitric, perchloric, or acetic acid, is identical with the mole. A normal solution of a monoprotic acid will therefore contain 1 mole per L of solution. The equivalent of a diprotic acid (e.g. sulphuric or oxalic acid), or of a triprotic acid (e.g. phosphoric( V) acid) is likewise one-half or one-third respectively, of the mole. 

To find the equivalent masses of the caustic soda and soda ash, they must be reacted with an acid, which may be represented by the hydrogen ion. Again, remember that the bases specified previously must first consume all the existing acidity of the water represented in the overall by H before they can raise the pH. Thus,... 

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