18 Questions
What is the relationship between pH and pOH values in a solution?
Their product is always 14
Which indicator is commonly used in pH paper to determine the pH of a solution?
Flavin
How does a pH meter determine the pH of a solution?
By interpreting electrical signals from a probe
According to Brønsted-Lowry theory, what defines an acid?
Any species that can donate a proton
How would you calculate the pH of a solution if provided with the hydrogen ion concentration in moles per liter?
$pH = -log[H^+]$
According to Lewis theory, what is a characteristic of a base?
Can accept an electron pair
What does a pH greater than 7 indicate in a solution?
Basic solution
How does the H+ ion concentration change when the pH decreases?
It increases
What is the function of flavin in pH paper?
Turns red in acidic solutions
What is used as an indicator in pH paper?
Flavin
Which method is more accurate for measuring pH than pH paper?
pH meter
What does a pH of 2 indicate about a solution?
Highly acidic
What is the relationship between pH and pOH in a water solution at 25°C?
pH + pOH = 14
What is the hydroxide ion concentration in a solution with a pOH of 9.25?
$1.78 \times 10^{-10}$ M
How can the hydrogen ion concentration be calculated from the pH of a solution?
[H+] = $10^{pH}$
What is the pH of a solution with a hydrogen ion concentration of $3.00 \times 10^{-8}$ M?
7.52
If a solution has a pH of 3.89, what is its hydroxide ion concentration?
$1.26 \times 10^{-11}$ M
What is the pOH of a solution with a hydroxide ion concentration of $5.62 \times 10^{-6}$ M?
$7.25$
Study Notes
pH and pOH
- Acidity and basicity of a substance are measured in terms of the molar concentration of hydrogen (H+) or hydroxide (OH-) ions, respectively.
- pH and pOH denote the concentration of H+ and OH- ions, respectively.
pH Scale
- pH scale allows classification of substances based on their acidity and basicity at 25°C.
- Substances with pH 7 are neutral, pH > 7 are basic, and pH < 7 are acidic.
- pH has an inverse relationship with H+ ion concentration.
Measuring pH
- pH paper measures pH by changing color depending on H+ and OH- ion concentration.
- pH paper is treated with flavin, an anthocyanin commonly found in red cabbages, which turns:
- Red in acidic solutions
- Greenish in neutral solutions
- Purple in basic solutions
- pH meter is a more accurate method than pH paper.
- pH meter contains a probe with two electrodes:
- Sensor electrode (pH 7 buffer solution)
- Reference electrode (saturated KCl solution)
Acid and Base Theories
Arrhenius Theory
- Acid: increases H+ ion concentration in aqueous solution
- Base: increases OH- ion concentration in aqueous solution
Brønsted-Lowry Theory
- Acid: donates a proton (H+)
- Base: accepts a proton
- Conjugate acid: formed after base accepts a proton
- Conjugate base: formed after acid donates a proton
Lewis Theory
- Acid: accepts a pair of non-bonding electrons
- Base: donates a pair of non-bonding electrons
- Acid-base complex: formed by reaction between Lewis acid and Lewis base
Calculating pH and pOH
- pH = - log [H+]
- [H+] = 10^(-pH) or [H+] = antilog (-pH)
- pOH = - log [OH-]
- [OH-] = 10^(-pOH) or [OH-] = antilog (-pOH)
Example Calculations
- Calculate pH of 0.0025 M HCl solution: pH = 2.60
- Calculate hydrogen ion concentration in a solution with pH 8.34: [H+] = 4.57 x 10^(-9) M
- Calculate pOH of a solution with hydroxide ion concentration 4.82 x 10^(-5) M: pOH = 4.32
- Calculate hydroxide ion concentration in a solution with pOH 5.70: [OH-] = 2.00 x 10^(-6) M
Test your knowledge on how to calculate pH and hydrogen ion concentration in solutions using mathematical expressions. Practice solving problems involving the relationship between pH, hydrogen ion concentration, and pOH.
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