Chapter 15, Problem 15.20QP

Strong Acids, Weak Acids, and pH

Two 0.10-mol samples of the hypothetical monoprotic acids HA(aq) and HB(aq) are used to prepare 1.0-L stock solutions of each acid.

1. a Write the chemical reactions for these acids in water. What are the concentrations of the two acid solutions?
2. b One of these acids is a strong acid, and one is weak. What could you measure that would tell you which acid was strong and which was weak?
3. c Say that the HA(aq) solution has a pH of 3.7. Is this the stronger of the two acids? How did you arrive at your answer?
4. d What is the concentration of A⁻(aq) in the HA solution described in part c?
5. e If HB(aq) is a strong acid, what is the hydronium-ion concentration?
6. f In the solution of HB(aq), which of the following would you expect to be in the greatest concentration: H₃O⁺(aq), B⁻(aq), HB(aq), or OH⁻(aq)? How did you decide?
7. g In the solution of HA(aq), which of the following would you expect to be in the greatest concentration: H₃O⁺(aq), A+(aq), HA(aq), or OH⁻(aq)? How did you decide?
8. h Say you add 1.0 L of pure water to a solution of HB. Would this water addition make the solution more acidic, make it less acidic, or not change the acidity of the original solution? Be sure to fully justify your answer.
9. i You prepare a 1.0-L solution of HA. You then take a 200-mL sample of this solution and place it into a separate container. Would this 200 mL sample be more acidic, be less acidic, or have the same acidity as the original 1.0-L solution of HA(aq)? Be sure to support your answer.

Interpretation Introduction

Interpretation:

The explanations for the given set of statements have to be given.

Concept introduction:

Acid:

According to Bronsted-Lowry theory, a species which donates a proton in a proton transfer-reaction is said to be an acid.

Base:

According to Bronsted-Lowry theory, a species which accepts a proton in a proton transfer-reaction is said to be base.

$\text{pH}$ is an logarithmic expression to express a solution is acidic, basic or neutral.  $\text{pH}$ scale lies values between 1-14 and on which 7 is neutral, below 7 values are more acidic in nature and above 7 values are more basic in nature.

Answer to Problem 15.20QP

(a)

The concentration for the two solutions is $\text{0}\text{.10 M}\text{.}$

The chemical equations for the given reactions are

${\text{HA}}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l)}}\stackrel{}{\to }{\text{ H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{ + A}}_{{}^{\text{-}}\text{(aq)}}$

${\text{HB}}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l)}}\stackrel{}{\to }{\text{ H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{ + B}}^{\text{-}}{}_{\text{(aq)}}$

(b)

The $\text{pH}$ of the strong acid solution is lower than the $\text{pH}$ of the weak acid solution.

(c)

The given solution is weaker of the two acids.

(d)

The concentration of ${\text{A}}^{\text{ -}}{}_{\text{(aq)}}$ is ${\text{2×10}}^{\text{-4}}\text{M}$

(e)

The concentration of hydronium ion is ${\text{[H}}_{\text{3}}{\text{O}}^{\text{+}}{\text{] = [B}}^{\text{-}}\text{] = 0}\text{.10 M}\text{.}$

(f)

The greatest concentration must be for ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\text{ion}\text{.}$

(g)

The greatest concentration must be for $\text{HA}\text{.}$

(h)

The $\text{pH}$ of the solution goes up.

(i)

The given solution has the same $\text{pH}$ of the original solution.

Explanation of Solution

(a)

The concentration for the two solutions is $\text{0}\text{.10 M}\text{.}$

The chemical equations for the given reactions are

${\text{HA}}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l)}}\stackrel{}{\to }{\text{ H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{ + A}}_{{}^{\text{-}}\text{(aq)}}$

${\text{HB}}_{\text{(aq)}}{\text{ + H}}_{\text{2}}{\text{O}}_{\text{(l)}}\stackrel{}{\to }{\text{ H}}_{\text{3}}{\text{O}}^{\text{+}}{}_{\text{(aq)}}{\text{ + B}}^{\text{-}}{}_{\text{(aq)}}$

(b)

By measuring the $\text{pH}$ of each solution, the $\text{pH}$ of the strong acid solution is lower than the $\text{pH}$ of the weak acid solution.

(c)

The given solution is weaker of the two acids.  Why because the strong acid has $\text{pH}$ of 1.00 but the given solution has $\text{pH}$ of 3.7.

(d)

The concentration of ${\text{A}}^{\text{ -}}{}_{\text{(aq)}}$ ion can be determined as follows.

${\text{[A}}^{\text{-}}{\text{] = [H}}_{\text{3}}{\text{O}}^{\text{+}}{\text{] = 10}}^{\text{-pH}}{\text{ = 10}}^{\text{-3}\text{.7}}={\text{2×10}}^{\text{-4}}\text{M}\text{.}$

Hence, the concentration of ${\text{A}}^{\text{ -}}{}_{\text{(aq)}}$ is ${\text{2×10}}^{\text{-4}}\text{M}\text{.}$

(e)

The concentration of hydronium ion is ${\text{[H}}_{\text{3}}{\text{O}}^{\text{+}}{\text{] = [B}}^{\text{-}}\text{] = 0}\text{.10 M}\text{.}$  Why because $\text{HB}$ is a strong acid.

(f)

The greatest concentration must be for ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\text{ion}\text{.}$  Why because $\text{HB}$ is a strong acid ${\text{[H}}_{\text{3}}{\text{O}}^{\text{+}}{\text{] > [OH}}^{\text{-}}\text{]}\text{.}$  The concentration of the strong acid $\text{HB}$ should be zero.  The formation of hydronium and ${\text{B}}^{\text{-}}$ ions form acid should be in equal numbers, so there will be some hydronium ions in water and it will slightly higher the concentration of ${\text{B}}^{\text{-}}.$

(g)

The greatest concentration must be for $\text{HA}\text{.}$  Why because $\text{HA}$ is weak acid and there must be more $\text{HA}$ than ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\text{or}{\text{A}}^{\text{-}}\text{ions}$.

(h)

The $\text{pH}$ of the solution goes up.  Why because while adding water to the $\text{HB}$ solution its concentration of acid could decrease by the factor of one-half.  Hence, the $\text{pH}$ of the solution will increase and the concentration goes down.  So, decrease in concentration of then the $\text{pH}$ increase.

(i)

The given solution has the same $\text{pH}$ of the original solution.  Why because the $\text{pH}$ of the solution must depends on concentration but not the volume which is going to test.

Conclusion

The explanations for the given set of statements were given.