Explain the following in terms of acid-base theory and principles. The K_a value

Question

Explain the following in terms of acid-base theory and principles. The K_a value for HCl is 3 times 10^-8, whereas the K_a value for HClO_3 is 1. Using phenolphthalein (pK_a = 9.3) rather than methyl orange (pK_a = 3.7) as the indicator for a titration where hydrochloric acid is added to a solution of sodium hydrogen carbonate produces an erroneous end-point. The aqueous salts produced in neutralization reactions between HCl_(aq) & NaOH_(aq) and HCl_(aq) & NH_3(aq), respectively, are found to have very different pH's. It is inappropriate to use an Erlenmeyer flask to measure out a volume of a standardized solution for use in a titration. Bases can be defined in at least three different ways.

Explanation / Answer

a)

Strong acids like HCl have higher Ka values close to 1, whereas weak acids have low Ka values.

HClO is a weak acid.

HClO3 is a strong acid.

b)

While doing titrations, the equivalence point must be contained in the same range of pH values over which the indicator is chromatically active. In other words, phenolphthalein should only be used if the titration system has an equivalence point that occurs between a pH of about 8 and 10. When HCl is added to sodium carbonate, the equivalence point will occur at pH < 7.

c)

Neutralization between HCl and NaOH will produce Na+, Cl- and water. pH will be close to 7 when equal amounts of HCl and NaOH are added.

HCl + NaOH --> NaCl + H2O

Neutralization between HCl and NH3 will produce NH4Cl salt. pH will be < 7 when equal amounts of HCl and NH3 are added.

NH4+ --> NH3 + H+

pH will be governed by Ka value of NH4+

d)

Erlenmeyer flasks are used for mixing, transporting, and reacting, but not for accurate measurements. The volumes stamped on them are approximate and accurate to within about 5%. Graduated cylinders, pipet and volumetric flasks are useful for measuring liquid volumes to good accuracy.

e)

Bases can be defined as:

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