pH Basics
This scale is used to measure how acidic or basic a solution is.
What is the pH scale?
An acid does this (increases or decreases) the hydrogen ion concentration in solution.
What is increases [H⁺]?
Water molecules can split into these two ions.
What are H⁺ (or H₃O⁺) and OH⁻?
If [H⁺] = 1.0 × 10⁻⁷ M, the pH is:
What is 7.00?
This tool is commonly used for quick pH estimation by color change.
What is pH paper/indicator paper?
Solutions with pH below 7 are classified as this.
What are acids?
A base does this to [H⁺].
What is decreases [H⁺] (by accepting H⁺ or producing OH⁻)?
Pure water has equal concentrations of these two ions.
What are [H⁺] and [OH⁻]?
If pH = 3.00, [H⁺] is:
What is 1.0 × 10⁻³ M?
If an enzyme stops working after pH becomes more acidic, the best explanation usually involves changes to this in the protein.
What is the enzyme’s shape/active site (from altered ionic/H-bond interactions)?
A solution with pH exactly 7 is considered this.
What is neutral?
This is the name for a compound that can act as either an acid or a base depending on conditions.
What is amphoteric (or amphiprotic)?
At 25°C, the ion product of water (Kw) is this value.
What is 1.0 × 10⁻¹⁴?
Compare pH 2 vs pH 5: the pH 2 solution has how many times more [H⁺]?
What is 10³ = 1000 times more?
You add a small amount of acid to a buffered solution and the pH barely changes. What property explains this?
What is buffering capacity (the buffer neutralizes added H⁺)?
Each 1-unit change in pH represents this kind of change in hydrogen ion concentration.
What is a 10-fold change?
A buffer resists changes in pH by doing these two actions when acid or base is added.
What is absorbing excess H⁺ and releasing H⁺ as needed (or absorbing OH⁻/supplying H⁺)?
If [H⁺] increases, what must happen to [OH⁻] in water (increase or decrease)?
What is decrease (because [H⁺][OH⁻] = Kw)?
If [OH⁻] = 1.0 × 10⁻⁵ M at 25°C, find pOH and pH.
What is pOH = 5.00 and pH = 9.00?
A student rinses a pH electrode with tap water and immediately inserts it into a sample without blotting. How could this affect the reading?
What is it can dilute/contaminate the sample at the electrode tip and skew pH toward the rinse water’s pH?
A solution changes from pH 6 to pH 4. Is it correct to say it became “twice as acidic”? Explain quantitatively.
What is no, it became 100 times more acidic because each pH unit represents a 10-fold change (10² = 100)?
A buffer is made of a weak acid (HA) and its conjugate base (A⁻). If a strong base (OH⁻) is added, which component reacts with it and what is formed?
What is the weak acid (HA) reacts with OH⁻ to form water (H₂O) and the conjugate base (A⁻)?
At 25°C, if [H⁺] in a solution is 3.2 × 10⁻⁶ M, calculate the pH to two decimal places.
What is pH = 5.49?
(−log(3.2 × 10⁻⁶) ≈ 5.49)
A solution has pH 8.30. Calculate the hydrogen ion concentration in scientific notation.
What is [H⁺] = 5.0 × 10⁻⁹ M?
(10⁻⁸·³ ≈ 5.0 × 10⁻⁹)
A student adds a large amount of strong acid to a buffered solution and observes a dramatic pH drop. What is the most likely explanation?
What is the buffer capacity was exceeded (all available conjugate base was used up), so the solution could no longer resist pH change?