CHAPTER 9

Acids, Bases, and Salts

Section 1    Section 2   Section 4    Section 5   Section 6

Section 7    Section 8   Section 9   Section 13

1.  An Arrhenius acid is any substance that when dissolved in water dissociates (breaks apart) to produce hydrogen ions (H⁺).  While substances that produce hydroxide ions (OH^-) when they dissociate in water are called Arrhenius bases.

a)  When dissolved in water, an Arrhenius ________ produces OH^- ions, while an Arrhenius ________ produces H⁺ ions.

b)  Label each of the following as an Arrhenius acid, base, or neither.

1)  HBr

2) Ca(OH)₂

3) Na₂CO₃

 Check Answers

2.  Any substance that contains hydrogen and is capable of donating a proton (H⁺) to another substance is called a Bronsted acid.  Any substance that is capable of accepting that proton is called a Bronsted base.

HCO₃^- + NH₄⁺ <==> H₂CO₃ + NH₃

For example, identify the Bronsted acid and the Bronsted base in both the forward and reverse reactions of the equilibrium above.

The forward reaction is:

HCO₃^- + NH₄⁺ → H₂CO₃ + NH₃

By inspection, it can be seen that NH₄⁺ loses a proton (an H⁺), making it the Bronsted acid:

NH₄⁺ → NH₃ + H⁺

It can also be seen that HCO₃^- gains a proton (an H⁺), making it the Bronsted base:

HCO₃^- + H⁺ → H₂CO₃

The reverse reaction is:

H₂CO₃ + NH₃ → HCO₃^- + NH₄⁺

Once again, inspection shows us that H₂CO₃ loses a proton, making it a Bronsted acid:

H₂CO₃ → HCO₃^- + H⁺

It can also be seen that NH₃ gains a proton, making it the Bronsted base:

NH₃ + H⁺ → NH₄⁺

Fill in the blanks with the chemical formulas to identify the Bronsted acids and bases in the following equation:

  a)  In the forward reaction, the acid is ________ and the base is ________.

b)  In the reverse reaction, the acid is ________ and the base is ________.

c)  In the reaction HCl + H₂O <==> H₃O⁺ + Cl^-, Cl^- is a Bronsted (acid/base/neither).

3.  When a Bronsted acid donates a proton (H⁺), the ion or substance that remains is called a conjugate base. 

For instance, what is the conjugate base for the Bronsted acid H₂CO₃?

Bronsted acids are anything that can give an H⁺ ion (a proton) to another compound.  When the acid loses the H⁺, it forms a conjugate base:

acid → H⁺ + base

So, to find the conjugate base for a Bronsted acid, simply remove an H from the formula and decrease the charge by one (-1) to account for the loss of the positive charge on the H⁺:

H₂CO₃ → H⁺ + HCO₃^-

Therefore, the conjugate base for H₂CO₃ is HCO₃^-.

Identify the conjugate base for each of the following Bronsted acids (in other words, write down what remains after the H⁺ has been donated):

a)     HNO₃

b)    HClO₄

4.  A Bronsted acid and its conjugate base can be grouped together as a conjugate acid-base pair.  For instance, in the equation:

 NH₃ + H₂O <==> NH₄⁺ + OH^-

NH₄⁺ and NH₃ are conjugate acid-base pairs, as are H₂O and OH^-.

HClO + H₂O <==> ClO^- + H₃O⁺

For the forward reaction:  On the reactant side of the equation above, the acid is (give formula) with its conjugate base (give formula) on the product side of the equation.

For the reverse reaction: On the product side of the equation above, the acid is (give formula) with its conjugate base (give formula) on the reactant side of the equation.

5.  Water molecules can self-ionize and form an equilibrium which abides by Le Châtelier’s principle:

H₂O + H₂O <==> H₃O⁺ + OH^-

If more H₃O⁺ is added to the water, the equilibrium shifts to the left by forming more (give formula).

6.  According to Le Châtelier’s principle, if more H₃O⁺ is added to water, the equilibrium shifts to the left forming more H₂O:

 H₂O + H₂O <==> H₃O⁺ + OH^-

If more water molecules are formed by the combining of H₃O⁺ and OH^-, the OH^- ions must be getting used up, and, therefore, the concentration of OH^- decreases when H₃O⁺ is added.

If OH^- ions are added to water, the H₃O⁺ concentration will (increase/decrease).

 Check Answers

7.  The term “neutral” is one used to describe any water solution in which the H₃O⁺ and OH^- concentrations are equal.  Neutral also refers to water solutions with a pH of 7 (see Section 9.5).

Water solutions with equal concentrations of H₃O⁺ and OH^- ions are considered ________ and have a pH of _______.

8.  A neutral water solution has an H₃O⁺ concentration (or [H₃O⁺]) of 1 x 10^-7 moles/L.  Therefore, the OH^- concentration (or [OH^-]) is also 1 x 10^-7 moles/L.  We can calculate an equilibrium constant for the dissociation of pure water into H₃O⁺ and OH^-:

K_w = [H₃O⁺][OH^-] = (1 x 10^-7 moles/L)(1 x 10^-7 moles/L)

K_w = 1 x 10^-14 (moles/L)²

Where K_w is referred to as the ion product of water.

Because it is an ________ constant, whenever [H₃O⁺] and [OH^-] are multiplied together the answer will always be 1 x 10^-14(moles/L)².  The product of [H₃O⁺][OH^-] is called the ________ product of water.

 Check Answers

9.  As noted earlier, when the concentration of either the H₃O⁺ ion or the OH^- ion increases, the concentration of the other ion must decrease.  An acidic solution is defined as a solution where the [H₃O⁺] is greater than the [OH^-]^.  Acidic solutions also have a pH value less than 7.  On the other hand, a basic or alkaline solution has an [OH^-] greater than the [H₃O⁺].  Basic solutions have a pH value greater than 7.

Classify each of the following as acidic, basic or neutral:

a)     [H₃O⁺] = 1 x 10^-10 moles/L, [OH^-] = 1 x 10^-4 moles/L

b)    [H₃O⁺] = 1 x 10^-2  moles/L, [OH^-] = 1 x 10^-12 moles/L

c)     [H₃O⁺] = 1 x 10^-7  moles/L, [OH^-] = 1 x 10^-7 moles/L

d)    pH = 8

e)     pH = 3

10.  Because K_w is a constant (1 x 10^-14(moles/L)²), we can determine the concentration of one of the ions (H₃O⁺, OH^-) if the concentration of the other is already known (see Example 9.4):

K_w = [H₃O⁺][OH^-]

Calculate the concentration of the ions whose concentration is not given.  Then classify the solutions as acidic, basic or neutral.  You'll notice that when you have an [H₃O⁺] of 1 x 10^x and an [OH^-] of 1 x 10^y that x + y always equals -14.  This is only true when there are 1's before the x 10^exp.

a)     [H₃O⁺] = 1 x 10^-8 moles/L, [OH^-] = ________ moles/L (acidic, basic or neutral).

b)    [OH^-] = 1 x 10^-2 moles/L, [H₃O⁺] = ________ moles/L (acidic, basic, neutral).

11.  pH is a scale of measurement used to express the acidity, basicity, or neutrality of a solution, where 7 is neutral, <7 is acidic, and >7 is basic.  Mathematically, pH is equal to the negative logarithm of the molar concentration of H⁺(H₃O⁺) in a solution.  In the following equations, we introduce the common practice of substituting H⁺ for H₃O⁺.

pH = -log[H⁺] or [H⁺] = 1 x 10^-pH

Looking at Table 9.1 in your text, the most acidic solutions have a pH = ________ and an [H⁺] of 1 x 10 (fill in exponent) moles/L.  The most basic solutions have a pH = ________ and an [H⁺] of 1 x 10 (fill in exponent) moles/L.

12.  Following the calculator procedure in Table 9.3 of your text, change the following pH values to [H⁺].

a)     pH = 3, [H⁺] =

b)    pH = 6.5, [H⁺] =

TI Calculator Instructions

13.  Now, follow the calculator procedures in Table 9.3 of your text to change the following pH values to [H⁺].  Then follow the procedures you used in question 10 to calculate the [OH^-].  Classify the solutions as acidic, basic, or neutral (only go one place past the decimal in the multiplier).

          1   a)  pH = 8, [H⁺] =

               b)  [OH^-] =

               c)  (acidic, basic, neutral)

          2   a)  pH = 11.3, [H⁺] =

               b)  [OH^-] =

               c)  (acidic, basic, neutral)

3   a) pH = 5, [H⁺] =

     b) [OH^-] =

     c) (acidic, basic, neutral)

4.  Suppose you have an [OH^-] of 6.5 x 10^-5 mol/L.  What is the H⁺ concentration (use the equation in question #10)?  What is the pH of the solution?

 Check Answers

14.  All acids have certain properties in common.  For instance, they all taste sour and produce H₃O⁺ ions when dissolved in water.  Acids also undergo double-replacement reactions with solid oxides (metal + oxygen), hydroxides (OH^-), carbonates (CO₃^-2), and bicarbonates (HCO₃^-) producing water as one of the products. 

Use equations 9.13 – 9.16 in your text as templates to fill in the missing products in the equations below.  Also state whether the acid is reacting with an oxide, hydroxide, carbonate, or bicarbonate.

a)     2HNO₃ + CuCO₃ ^→ Cu(NO₃)₂ + ________ + H₂O

b)    (oxide, hydroxide, carbonate, or bicarbonate)

c)     H₂SO₄ + Mg(OH)₂ ^→ MgSO₄ + 2 ________

d)    (oxide, hydroxide, carbonate, or bicarbonate)

15.  Another property of acids is their ability to react with certain metals to make hydrogen gas:

Mg + 2HCl ^→ MgCl₂ + H₂

a)  Identify which of the following will produce hydrogen gas, carbon dioxide gas, and/or water when reacted with HCl, hydrochloric acid.

1)  CaO

2) Zn(OH)₂

3) Zn

4) CaCO₃

5) NaHCO₃

b)  Following text Example 9.8, write the following equations in total ionic form, net ionic form and identify any spectator ions.  Note that only the compounds designated with an (aq) symbol completely dissociate into ions (See question #14 - #18 in chapter 5 of this tutor for a review).

1)  2HBr_(aq) + Pb_(s) → PbBr_2(s) + H_2(g)

2)  H₂SO_4(aq) + 2NaOH_(aq) → 2H₂O_(l) + Na₂SO_4(aq)

 Check Answers

16.  When an acid and a base react completely leaving only a salt and water, the reaction is known as a neutralization reaction.

In a neutralization reaction, an acid and a base completely react forming water and a ________.

17.  In general, the term salt refers to the ionic compound left over after the H⁺ of an acid and the OH^- of a base react to form water.  Salt does not always mean NaCl (table salt).

Following equations 9.21 – 9.24 in your text, complete the molecular, total ionic, and net ionic equations representing the neutralization reaction of HF (hydrofluoric acid) and LiOH.  Be sure to include any coefficients other than one in your answers.

Molecular:  (a)  + HF ^→  (b)  + H₂O

Total Ionic:  (c)  + OH^- + H⁺ +  (d)  ^→  (e)  + F^- +  (f)

Net Ionic: OH^- +  (g)  ^→  (h)

Check Answers 

18.  Ions are categorized according to their charge.  A cation is a positively charged ion while a negatively charged ion, is known as an anion. An easy way to remember this is that ca”t”ion has a “+” sign in the spelling.

Identify the cations and anions of the following salts by giving the ion symbol and charge.  You may want to review text Table 4.7.

1)     NaCl. cation =  (a)  anion =  (b)

2)     NH₄NO_3. cation =  (c)  anion = (d)

19.  Salts are ionic compounds that contain the cation of a base and an anion from an acid.  Salts form a solid crystalline structure at room temperature.

a)  Salts contain positively charged ions that come from a(n) ________ and negatively charged ions that come from a(n) ________.

b)  What salt would be made with each of the following reactions?

1)  H₂SO₄ + NaOH →

2)  HBr + LiOH →

3)  HCl + NaHCO₃ →

20.  Salts can be prepared by reacting a solution of an appropriate acid with a metal, a metal oxide, a metal hydroxide, a metal carbonate, or a metal bicarbonate (see Equations 9.26 – 9.30).

Complete the general reactions below by filling in the names of the missing compounds (each type of reaction is only represented once).

a)     acid + ________ ________ ^→ salt + water

b)    acid + ________ ^→ salt + hydrogen

c)     acid + metal bicarbonate ^→ salt + water + ______

d)    acid + metal hydroxide ^→ _______ + water

e)     acid + _____ _____ ^→ salt + _____ + carbon dioxide

21.  When salts dissolve in water, the cations and anions usually dissociate completely to form a solution.  Strong acids and bases behave the same, dissociating completely when dissolved.  However, there are some acids and bases that do not dissociate completely in water and are therefore known as weak or moderately weak acids and bases.

Using Table 9.7, rank the following acids according to their % dissociation (1= most dissociation, 3= least dissociation) and classify them as strong, moderately weak, or weak acids.

1)     H₂CO₃ Rank  (a) , classification  (b)

2)     HBr  Rank  (c) , classification  (d)

3)     H₂SO₃  Rank  (e) , classification  (f)

22.  One way acids can be categorized is by the number of protons (H⁺) per molecule that can dissociate.  Once again, we refer back to the Greek prefixes mono-, di-, and tri-.  Hence, a monoprotic acid can lose only one proton per molecule when dissolved, diprotic acid, two protons, and tri-protic, three.  Refer to Table 9.8 for examples of mono-, di-, and tri-protic acids.

Identify the following acids as monoprotic, diprotic, or triprotic.

a)     H₂SO₄ classification ________

b)    HBr     classification ________

c)     H₃BO₃ classification ________

Check Answers 

23.  A solution that resists changes in pH when acids or bases are added is known as a buffer.

A solution whose pH remains constant when OH^- is added is likely a(n) ________.

24.  Although buffers can be very effective when it comes to resisting changes in pH, if too much acid (H⁺) or base (OH^-) is added to the buffer solution, the pH will change.  The amount of acid or base that can be added to a buffer without significantly changing the pH is known as the buffer capacity.

A buffer solution has a pH of 4.3.  To increase the pH to 5.1 a significant amount of (acid/base) must be added.  The amount added must exceed the _____ ______ of the buffer.

Check Answers

Home Page