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Chapter 15: Problem 41

Why do we normally not quote \(K_{\mathrm{a}}\) values for strong acids such as \(\mathrm{HCl}\) and \(\mathrm{HNO}_{3}\) ? Why is it necessary to specify temperature when giving \(K_{\mathrm{a}}\) values?

Short Answer

Expert verified
We don't quote \(K_{\mathrm{a}}\) values for strong acids like \(\mathrm{HCl}\) and \(\mathrm{HNO}_{3}\) because they completely dissociate in water, hence their \(K_{\mathrm{a}}\) values would be extremely large. It's necessary to specify the temperature when giving \(K_{\mathrm{a}}\) values because these values are temperature-dependent, and an increase in temperature results in a larger \(K_{\mathrm{a}}\) value.

Step by step solution

01

Understanding Acid Strength and \(K_{\mathrm{a}}\) values

The strength of an acid is determined by its ability to donate protons (or \(H^{+}\) ions) to a solution. In water, strong acids like \(\mathrm{HCl}\) and \(\mathrm{HNO}_{3}\) completely dissociate into their ions, meaning nearly every molecule of these acids donate a proton to the solution. Therefore, their \(K_{\mathrm{a}}\) values would be extremely large and it's not necessary to quote these values. It's more relevant to quote \(K_{\mathrm{a}}\) values for weak acids, which only partially dissociate in water.
02

Temperature and \(K_{\mathrm{a}}\) Values

The value of \(K_{\mathrm{a}}\) (like all equilibrium constants) is temperature-dependent. This is why it's necessary to specify the temperature when giving \(K_{\mathrm{a}}\) values. An increase in temperature would increase the rate of the forward reaction (the dissociation of the acid), thus leading to a larger \(K_{\mathrm{a}}\) value as more products (ions) are formed. Therefore, \(K_{\mathrm{a}}\) values are only true at the specified temperature.

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Most popular questions from this chapter

The \(K_{\mathrm{a}}\) for benzoic acid is \(6.5 \times 10^{-5} .\) Calculate the \(\mathrm{pH}\) of a \(0.10 \mathrm{M}\) benzoic acid solution.

A \(0.400 M\) formic acid (HCOOH) solution freezes at \(-0.758^{\circ} \mathrm{C}\). Calculate the \(K_{\mathrm{a}}\) of the acid at that temperature. (Hint: Assume that molarity is equal to molality. Carry your calculations to three significant figures and round off to two for \(K_{\mathrm{a}}\).)

Determine the concentration of a \(\mathrm{NH}_{4} \mathrm{Cl}\) solution that has a pH of 5.64 .

\(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a strong acid, but \(\mathrm{HSO}_{4}^{-}\) is a weak acid. Account for the difference in strength of these two related species.

When chlorine reacts with water, the resulting solution is weakly acidic and reacts with AgNO \(_{3}\) to give a white precipitate. Write balanced equations to represent these reactions. Explain why manufacturers of household bleaches add bases such as \(\mathrm{NaOH}\) to their products to increase their effectiveness.
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