Chemistry: Acids, Bases and pH

Terms in this set (14)

What do Arrhenius acids produce in water?

H+ (hydronium, H3O+) ions

An Arrhenius base produces ___ ions in water.

OH-

What is a Brønsted-Lowry acid?

A proton (H+) donor

In the Brønsted-Lowry definition, what remains when an acid donates a proton?

Its conjugate base

Lewis acids are defined by their ability to accept ___.

electron pairs

Name three strong acids.

HCl, HBr, HI, HNO3, H2SO4, HClO4 (any three)

What does a larger Ka value indicate about an acid?

A stronger acid

The pH scale is defined as pH = ___.

-log[H+]

A solution at pH 3 has how many times the hydrogen ion concentration of a solution at pH 4?

Ten times

Neutralization

The reaction of an acid with a base to form water and a salt.

In a titration, the equivalence point is reached when ___.

moles of acid equal moles of base

What is the pH at the equivalence point when titrating a strong acid with a strong base?

pH 7

What kind of solution resists changes in pH when small amounts of acid or base are added?

A buffer

The Henderson-Hasselbalch equation is pH = pKa + log(___).

[A-]/[HA]

Practice questions (15)

1. 1.In the reaction `HCl + H2O → H3O+ + Cl-`, which of the options correctly identifies a Brønsted-Lowry conjugate acid-base pair?

   • AHCl is the conjugate acid of H2O.
   • BCl- is the conjugate acid of HCl.
   • CH3O+ is the conjugate acid of H2O.
   • DH2O is the conjugate base of Cl-.
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   Answer: H3O+ is the conjugate acid of H2O.

   This question checks the student's ability to apply the Brønsted-Lowry definition. H2O accepts a proton (H+) from HCl to become H3O+, making H3O+ the conjugate acid of the base H2O. HCl donates a proton, making Cl- its conjugate base.

2. 2.True or False: According to the Arrhenius definition, ammonia (NH3) is considered a base because it accepts a proton (H+) from water, increasing the concentration of hydroxide (OH-) ions.

   • ATrue
   • BFalse
   Show answer

   Answer: False

   This is a tricky question that requires careful distinction between definitions. While NH3 is a base and does increase OH- concentration, the Arrhenius definition specifically requires the base itself to produce OH- ions in water (like NaOH does). The description of NH3 as a proton acceptor fits the Brønsted-Lowry definition, not the Arrhenius one. This challenges the student to see the limitations of the Arrhenius model.

3. 3.A substance that produces OH- ions when dissolved in water is called an Arrhenius base. A substance that accepts a proton (H+) is a Brønsted-Lowry base. How can ammonia (NH3), which contains no hydroxide ions, be classified as both an Arrhenius base and a Brønsted-Lowry base?

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   Answer: Ammonia acts as a Brønsted-Lowry base by accepting a proton from a water molecule. This reaction, NH3 + H2O ⇌ NH4+ + OH-, produces hydroxide ions (OH-). Because its presence in water leads to the production of OH- ions, it also fits the Arrhenius definition of a base, even though the NH3 molecule itself doesn't contain OH-.

   This question forces a synthesis of two definitions. Students must connect the proton-accepting action (Brønsted-Lowry) with the resulting production of hydroxide ions in the solution (Arrhenius), demonstrating a deeper understanding of how the definitions relate and overlap.

4. 4.Which of the following statements correctly describes the difference between a strong acid like hydrochloric acid (HCl) and a weak acid like acetic acid (CH3COOH)?

   • AStrong acids have a higher pH than weak acids at the same concentration.
   • BStrong acids have a larger pKa value than weak acids.
   • CStrong acids are less reactive than weak acids.
   • DStrong acids dissociate completely in water, while weak acids only dissociate partially.
   Show answer

   Answer: Strong acids dissociate completely in water, while weak acids only dissociate partially.

   This question directly assesses the core concept of dissociation, which is the key difference between strong and weak acids as defined in the text. Understanding this is fundamental to all other acid-base calculations and concepts.

5. 5.A weak acid with a smaller pKa value is stronger than a weak acid with a larger pKa value.

   • ATrue
   • BFalse
   Show answer

   Answer: True

   This question checks the student's ability to interpret pKa values, which is a crucial skill for comparing the relative strengths of weak acids. Since pKa is a negative logarithmic scale, the inverse relationship between pKa and acid strength can be a point of confusion.

6. 6.Explain why titrating a weak acid with a strong base results in an equivalence point with a pH greater than 7, whereas titrating a strong acid with a strong base results in an equivalence point at pH 7.

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   Answer: When a weak acid is neutralized by a strong base, its conjugate base is formed. This conjugate base is weak and reacts with water (hydrolyzes) to produce hydroxide ions (OH-), making the solution basic at the equivalence point (pH > 7). In a strong acid-strong base titration, the salt formed consists of ions that do not hydrolyze water, so the solution is neutral (pH = 7).

   This question challenges students to connect the concepts of weak acids, conjugate bases, and hydrolysis to explain a key experimental observation in titrations. It requires synthesizing multiple ideas from the text to form a coherent explanation, demonstrating a deeper level of understanding beyond simple definitions.

7. 7.If a solution has a hydroxide ion [OH-] concentration of 1.0 x 10^-9 M, what is its pH at 25 °C?

   • A9
   • B5
   • C14
   • D7
   Show answer

   Answer: 5

   To find the pH, first calculate the pOH using the formula pOH = -log[OH-], which gives pOH = -log(1.0 x 10^-9) = 9. Then, use the relationship pH + pOH = 14 to find the pH. So, pH = 14 - 9 = 5. This question tests the ability to perform a two-step calculation involving both pOH and pH.

8. 8.A solution at pH 2 has a hydrogen ion [H+] concentration one hundred times greater than a solution at pH 4. True or false?

   • ATrue
   • BFalse
   Show answer

   Answer: True

   The pH scale is logarithmic, meaning each whole number change represents a tenfold difference in hydrogen ion concentration. Moving from pH 4 to pH 3 is a 10x increase in [H+], and moving from pH 3 to pH 2 is another 10x increase. Therefore, the total increase from pH 4 to pH 2 is 10 * 10 = 100 times.

9. 9.What is the hydrogen ion concentration, [H+], in a solution with a pOH of 11 at 25 °C?

   Show answer

   Answer: 1.0 x 10^-3 M

   This question requires a multi-step calculation. First, determine the pH using the relationship pH + pOH = 14. So, pH = 14 - 11 = 3. Next, calculate the hydrogen ion concentration from the pH using the inverse of the pH formula: [H+] = 10^-pH. Therefore, [H+] = 10^-3 M or 1.0 x 10^-3 M.

10. 10.During a titration, what is the 'equivalence point'?

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    Answer: The equivalence point is the point in a titration where the moles of the added titrant (the solution of known concentration) are chemically equivalent to the moles of the substance being analyzed, meaning the moles of acid equal the moles of base.

    This question checks for understanding of a key term in the process of titration. The equivalence point is the theoretical goal of a titration, and reaching it allows for the calculation of the unknown concentration, which is the purpose of the procedure.

11. 11.If you titrate a weak acid like acetic acid (CH3COOH) with a strong base like sodium hydroxide (NaOH), the equivalence point will be at a pH greater than 7. Which of the following best explains why?

    • AThe salt formed, sodium acetate, contains the conjugate base of the weak acid, which reacts with water (hydrolyzes) to produce OH- ions.
    • BThe strong base is much more concentrated than the weak acid.
    • CWeak acids do not fully neutralize, so the solution remains slightly acidic.
    • DThe indicator used for weak acid titrations only changes color in basic conditions.
    Show answer

    Answer: The salt formed, sodium acetate, contains the conjugate base of the weak acid, which reacts with water (hydrolyzes) to produce OH- ions.

    This question challenges the student to go beyond the simple definition of neutralization and apply their knowledge of conjugate bases and hydrolysis. Understanding why the equivalence point isn't always at pH 7 is crucial for accurately interpreting titration results for different types of acids and bases.

12. 12.In a neutralization reaction, an acid and a base react to form only water.

    • ATrue
    • BFalse
    Show answer

    Answer: False

    This question targets a common oversimplification. While water is a product, the formation of a salt is the other key component of a neutralization reaction. Recognizing this is fundamental to understanding the overall chemical transformation that occurs.

13. 13.A buffer solution is prepared using a weak acid (HA) and its conjugate base (A-). What happens when a small amount of a strong acid (like HCl) is introduced into this buffer?

    • AThe added H+ ions react with the weak acid (HA) to form more conjugate base (A-), causing the pH to decrease significantly.
    • BThe weak acid (HA) and the conjugate base (A-) are consumed completely, and the buffer stops working.
    • CThe buffer's pH remains absolutely unchanged because the added acid is instantly destroyed.
    • DThe added H+ ions are neutralized by the conjugate base (A-), forming more of the weak acid (HA), which results in only a very small change in pH.
    Show answer

    Answer: The added H+ ions are neutralized by the conjugate base (A-), forming more of the weak acid (HA), which results in only a very small change in pH.

    This question assesses the student's understanding of the fundamental mechanism of a buffer. The key principle is that the conjugate base component of the buffer neutralizes added acid, thus resisting a large drop in pH. This is a core concept for explaining how buffers work.

14. 14.A buffer solution is most effective at resisting pH changes when the pH of the solution is close to the pKa of the weak acid used to make it.

    • ATrue
    • BFalse
    Show answer

    Answer: True

    This question checks comprehension of a key property of buffers mentioned in the text. Understanding the relationship between a buffer's effective range and the acid's pKa is crucial for applying buffer concepts, such as in selecting an appropriate buffer for a specific application.

15. 15.What are the two essential components of a buffer solution that enables it to resist changes in pH?

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    Answer: A buffer solution consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.

    This question directly tests the student's knowledge of the composition of a buffer. Knowing the components is the first step to understanding the mechanism by which it resists pH changes. The presence of both an acidic and a basic species allows it to neutralize either added base or added acid.