pH and pOH of Strong Acids and Bases
A solution has a pH of 3. What is the pOH?
Answer: pOH = 11
Explanation & Work -
Use the formula: pH + pOH = 14
14 - 3 = 11
What is true about a weak acid in water?
A. It completely breaks apart
B. It partially breaks apart
C. It does not react with water
D. It turns into a base
Answer: B. It partially breaks apart
Explanation -
Weak acids only ionize a little in water, so an equilibrium forms between the reactants and products.
What happens in an acid base neutralization reaction?
A. Two acids react
B. Two bases react
C. An acid and base react to form water and salt
D. Water breaks apart
Answer: C. An acid and base react to form water and a salt
Explanation - Neutralization occurs when hydrogen ions from the acid react with hydroxide ions from the base.
H+ + OH- -> H2O
What is the purpose of an acid-base titration?
A. To boil a solution
B. To determine the concentration of an acid or base
C. To seperate mixtures
D. To measure temperature
Answer: B. To determine the concentration of an acid or base
Explanation - In a titration, a solution with a known concentration is added to a solution with an unknown concentration until neutralization occurs.
What makes an acid acidic?
A. It releases hydroxide ions
B. It releases hydrogen ions
C. It releases electrons
D. It absorbs water
Answer: B. It releases hydrogen ions
Explanation: Acids release H+ ions in water, which gives them their acidic properties.
A strong base has a pOH of 4. What is the pH?
Answer: pH = 10, which means the solution is basic.
Explanation -
Use the formula: pH + pOH = 14
14 - 4 = 10
Write the equilibrium expression for the weak acid reaction below:
HA ~ H+ + A-
* the ~ symbolizes double arrow equilibrium
Answer: Ka = [H+][A-]/[HA]
Explanation -
Ka measures how much a weak acid dissociates in water. A larger Ka means a stronger weak acid.
What are the products of this reaction?
HCl + NaOH -> ?
Answer: NaCl and H2O
Explanation - Hydrochloric acid reacts with sodium hydroxide to make salt and water.
HCl + NaOH -> NaCl + H2O
What is the equivalence point in a titration?
A. When the solution changes color
B. When all the liquid evaporates
C. When the acid and base have reacted in exact amounts
D. When the pH is always 7
Answer: C. When the acid and base have reacted in exact amounts
Explanation: At the equivalence point, the moles of acid equal the moles of base according to the balanced equation.
Why is HCl considered a strong acid?
A. It has many atoms
B. Its bond easily breaks apart in water
C. It contains oxygen
D. It is a solid
Answer: B. Its bond easily breaks apart in water
Explanation - The bond between hydrogen and chlorine is very polar, so HCl dissociates completely in water.
A strong acid has a hydrogen ion concentration of 1x10-5. What is the pH?
Answer: pH = 5
Explanation -
Use the formula: pH = log[H+]
Since the exponent is -5, the pH is 5.
A weak acid has a Ka = 1.0x10-5. Is this acid strong or weak?
Answer: It is a weak acid.
Explanation - Strong acids dissociate almost completely and have a very large Ka values. A small Ka means only a small amount ionizes, so the acid is weak.
What is a buffer?
A. A solution that always has a pH of 7
B. A solution that resists changes in pH
C. A strong acid
D. Pure water
Answer: B. A solution that resists changes in pH
Explanation - Buffers help keep pH stable even when small amounts of acid or base are added.
A student titrates 25.0mL of HCl with 0.100M NaOH. It takes 25.0mL of NaOH to reach the equivalence point. What is the concentration of the HCl?
Answer: 0.100M HCl
Explanation - Use the titration formula: M1V1 = M2V2
Substitute the values: M1(25.0) = 0.100(25.0)Solve: M1 = 0.100M
What structural feature is common in many bases?
A. They contain OH- ions
B. They always contain carbon
C. They have double bonds
D. They contain only metals
Answer: A. They contain OH- ions
Explanation - Many bases, such as NaOH and KOH, release hydroxide ions in water.
A strong base has a hydroxide ion concentration of 1x10-3. What are the pOH and pH?
Answer: pOH = 3, pH = 11
Explanation -
First use: pOH = -log[OH-]
The exponent is -3, so pOH = 3.
Then use: pH + pOH = 14
14 - 3 = 11
So the pH is 11.
A weak base has a Kb = 2.0x10-4. What does this tell you about the base?
Answer: The base partially ionizes and is moderately weak.
Explanation -
Kb measures how much a weak base dissociates in water.
Kb = [BH+][OH-]/[B]
Because the Kb value is less than 1, the base is weak.
A buffer is made from acetic acid and acetate ions. What happens if a small amount of acid is added?
Answer: The acetate ions react with the added acid, so the pH changes very little.
Explanation - Buffers work because one substance removes added acids while the other removes added bases.
CH3COO- + H+ -> CH3COOH
50.0 mL of 0.200M HNO3 is titrated with NaOH. How many moles of HNO3 are present?
Answer: 0.0100 moles
Explanation - Use the mole formula: n = MV
Convert volume to liters: 50.0mL = 0.0500 L
Now solve: n = (0.200)(0.0500) = (0.0100)
So there are 0.0100 moles of acid.
Why is acetic acid (CH3COOH) considered a weak acid?
Answer: It only partially ionizes in water.
Explanation: The molecular structure of acetic acid keeps most of the hydrogen atoms bonded, so only a small amount of H+ is released.
CH3COOH ~ H+ + CH3COO-
An equilibrium forms because the reaction does not fully proceed forward.
A strong acid has a hydrogen ion concentration of 3.2x10-4. What is the pH? *round to the nearest hundreth
Answer: pH = 3.49
Explanation -
Use: pH = -log[H+]
Substitute the value: pH = -log(3.2x10-4)
Using a calculator: pH = 3.49
Since the pH is below 7, the solution is acidic.
A weak acid solution has [H+] = 2.5x10-3 M. What is the pH?
*round to the nearest hundreth
Answer: pH = 2.60
Explanation -
Use the formula: pH = -log[H+]
Substitute the value: pH = -log(2.5x10-3)
Using a calculator: pH = 2.60
The solution is acidic because the pH is below 7.
A buffer solution has a pH of 4.75 and a pKa of 4.75. What does this tell you about the concentrations of the acid and conjugate base?
Answer: The concentrations are equal.
Explanation - Use the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA])
If pH = pKa, then: log([A-]/[HA]) = 0
This means: [A-]/[HA] = 1
So the acid and conjugate base concentrations are equal.
30.0mL of 0.150M H2SO4 is titrated with 0.100M NaOH. How many mL of NaOH are needed to completely neutralize the acid?
Answer: 90.0mL NaOH
Explanation - First write the balanced equation:
H2SO4 + 2NaOH -> Na2SO4 + 2H2O
Find moles of acid: n = (0.150)(0.0300) = 0.00450 mol
Because the ratio is 1:2 -
0.00450 x 2 = 0.00900 mol NaOH
Now use: V=n/M V = 0.00900/0.100 = 0.0900 L
Convert to mL: 0.0900 L = 90.0mL
So 90.0mL of NaOH are required.
Ammonia (NH3) is a weak base even though it does not contain OH-. Why?
Answer: It accepts hydrogen ions from water, producing hydroxide ions.
Explanation - According to the Bronsted-Lowry theory, bases accept protons.
NH3 + H2O ~ NH4+ + OH-