Unit notes and practice questions
Everything here is for unit 1 of the Grade 12 Chemistry textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.
The cards filed under this chapter on the Grade 12 Chemistry page. Open one at a time.
When you use it
Use when finding the pH of a buffer solution composed of a weak acid and its conjugate base.
Watch out
The ratio inside the logarithm must place the conjugate base concentration in the numerator and the weak acid in the denominator.
Drafted from Grade 12 Chemistry, pages 3-56, then checked twice before it went up
When you use it
Use when converting between hydronium and hydroxide ion concentrations or calculating pH and pOH in aqueous solutions at 25 °C.
Watch out
The value 1.0 times 10^-14 is strictly at 25 °C, and changes when temperature varies.
Drafted from Grade 12 Chemistry, pages 3-56, then checked twice before it went up
When you use it
Reach for this when simplifying equilibrium expressions for weak acids and weak bases by dropping x from initial concentration.
Watch out
If the ionized concentration x exceeds 5 percent of the initial acid concentration, the approximation fails and the quadratic formula must be used.
Drafted from Grade 12 Chemistry, pages 3-56, then checked twice before it went up
| Parent Acid | Parent Base | Solution Nature | pH Range at 25 °C |
|---|---|---|---|
| Strong | Strong | Neutral | |
| Weak | Strong | Basic | |
| Strong | Weak | Acidic | |
| Weak | Weak | Depends on and |
When you use it
Use to predict whether a dissolved salt yields an acidic, basic, or neutral aqueous solution.
Watch out
For salts of weak acid and weak base, compare Ka of the cation and Kb of the anion to decide acidity.
Drafted from Grade 12 Chemistry, pages 3-56, then checked twice before it went up
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
Under the Brønsted Lowry concept, an acid is a substance that
Answer: A donates a proton
Why
Brønsted Lowry defines an acid as a proton donor and a base as a proton acceptor. Option C is the Lewis definition of an acid and option D is the Arrhenius definition of a base.
Question 2
What is the conjugate base of H₂SO₄?
Answer: B HSO₄⁻
Why
A conjugate pair differs by exactly one proton. Removing one H⁺ from H₂SO₄ leaves HSO₄⁻. Option A is the conjugate base of HSO₄⁻, so it is two protons away and one step too far. Option C is the conjugate acid.
Question 3
What is the pH of a 0.001 mol/L solution of HCl?
Answer: C 3
Why
HCl is a strong acid and ionizes completely, so the hydrogen ion concentration is 0.001, which is 10⁻³. pH is the negative logarithm of that, so pH = 3. Option D is the pOH, and pH plus pOH equals 14.
Question 4
BF₃ has no proton to give, yet it is classed as an acid. Under which concept, and why?
Answer: C Lewis, because it accepts a pair of electrons
Why
Boron in BF₃ has only six electrons around it, so it readily accepts an electron pair. That is the Lewis definition of an acid, and it is exactly why the Lewis concept is the broadest of the three.
Question 5
The Ka of a weak acid is 2.0 x 10⁻⁵. What is the Kb of its conjugate base?
Answer: B 5.0 x 10⁻¹⁰
Why
For a conjugate pair, Ka multiplied by Kb equals Kw. So Kb = 1.0 x 10⁻¹⁴ divided by 2.0 x 10⁻⁵ = 5.0 x 10⁻¹⁰. Option C comes from multiplying the two values instead of dividing.
Question 6
A buffer contains a weak acid and its salt at equal concentrations. Its pH is
Answer: B equal to the pKa of the acid
Why
The Henderson Hasselbalch equation gives pH = pKa + log([salt]/[acid]). When the two concentrations are equal the ratio is 1 and log 1 is zero, so pH equals pKa. This is also the point at which the buffer resists change most effectively.
Question 7
An aqueous solution of ammonium chloride is
Answer: A acidic, because the ammonium ion hydrolyses
Why
Ammonium chloride comes from a strong acid, HCl, and a weak base, ammonia. The stronger parent decides, so the solution is acidic. The ammonium ion reacts with water and releases hydronium ions, while the chloride ion is too weak a base to do anything.
Question 8
Which indicator is most suitable for titrating acetic acid with sodium hydroxide?
Answer: B Phenolphthalein
Why
A weak acid with a strong base has its equivalence point above pH 7, because the salt formed hydrolyses to give a basic solution. Phenolphthalein changes between pH 8.3 and 10.0, which sits inside the steep part of that curve. Methyl orange would change far too early.
Question 9
Adding sodium acetate to a solution of acetic acid causes the ionization of the acid to decrease. This is
Answer: A the common ion effect
Why
Sodium acetate supplies acetate ions, which the acid already produces. By Le Chatelier's principle the extra acetate pushes the ionization equilibrium back to the left, so fewer hydronium ions form. That suppression is the common ion effect, and it is the basis of how a buffer is made.
Question 10
Water at 50 degrees Celsius has a pH slightly below 7. What does this tell you?
Answer: B The water is still neutral, because Kw rises with temperature
Why
Neutral means the hydronium and hydroxide concentrations are equal, not that the pH reads exactly 7. Kw increases with temperature, so both concentrations rise and the neutral pH falls below 7 while remaining neutral. The value 7 only applies at 25 degrees Celsius.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
Unit 1 runs from page 3 to page 56 of the Ministry of Education edition.
Ministry of Education edition, English, 298 pages
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