⏱️ June 10, 2020
pH and pOH are topics in unit 8, and AP Chemistry as a whole, that are extremely important. So then, what is pH? pH simply is a measure of the concentration of protons in a solution.
Essentially, pH is measured by the concentration of H+ ions, meaning it measures how acidic a solution is. A higher pH means a lower concentration of H+, implying a more basic solution and vice versa (I know, that's really annoying). So we get why there's the H in pH - H means hydrogen, pH measures the concentration of hydrogen ions - but what's that p? In chemistry, 'p' is a symbol that means negative log. p(anything) = -log(anything). Thus, pH = -log([H+]). For example, if we had a solution that had 0.01M H+, that would mean pH = -log(1 * 10^-2) = -(-2) = 2 (note that log() implies log base 10).
If pH = -log([H+]), then one can logically find that pOH = -log([OH-]) and is in essence the opposite of pH. Where pH is measured with the concentration of H+, pOH is measured with the concentration of OH-. pOH measures how basic a solution is. A low pOH means a highly basic solution and vice versa.
Let's suggest we had the reaction OH- + H+ --> H2O. This can be described as the protonation of OH-, or the autoionization of water. This is an important reaction, as it also serves as the net ionic for a strong acid-strong base interaction. This reaction can further be described using equilibrium by flipping the reaction to say that H2O ⇌ OH- + H+. The K value for this reaction is a constant called Kw. Kw = 1 * 10^-14.
Let's think about this reaction a little bit. From Kw that we were given before, we can write that Kw = [OH-][H+]. Therefore, 10^-14 = [OH-][H+]. Now, let's take the negative log of both sides:
14 = -log([OH-][H+]) = -log([OH-]) + -log([H+])
From here it is clear then that pH + pOH = 14.
For these examples, we're going to be looking just at pH since that's the most common measure of acidity, but remember, pOH can be easily calculated from this. When dealing with strong acids (and by extension strong bases), what's important is that these reactions go fully forwards. That is to say, they have a K value so high that to call it an equilibrium is negligable. Therefore, all of the acid/base will dissociate. Let's take a look at an example.
Suppose we have a 1M solution of HCl. The dissolution of HCl looks like: HCl --> H+ + Cl- (we're gonna stick with the Arrhenius definition of acids to make things simpler). Because this reaction goes completely forward, we know that 1M of HCl will dissolve into 1M of H+ and 1M of Cl-. To find pH, we take the -log([H+]).
pH = -log(1) = -0 = 0. Therefore a 1M solution of HCl has a pH of 0.
The same idea applies to strong bases. Suppose we had 1M NaOH. NaOH --> Na+ + OH-, so pOH = -log([OH-]) = -log(1) = 0.
To find pH we then plug into pH + pOH = 14, which tells us that the pH of a 1M solution of NaOH is 14.
There are 7 strong acids you need to memorize for AP Chemistry. Luckily, they're not that bad and they become second nature after a while. Here is the list:
✍️ Free Response Questions
AP Chemistry Free Response Questions
⚛️ Unit 1: Atomic Structure and Properties
1.1Moles and Molar Mass
1.2Mass Spectroscopy of Elements
1.3Elemental Composition of Pure Substances
1.4Composition of Mixtures
1.5Atomic Structure and Electron Configurations
1.6Photoelectron Spectroscopy & Graph Interp.
🤓 Unit 2: Molecular and Ionic Compound Structures and Properties
2.0Unit 2 Overview: Molecular and Ionic Bonding
2.1Types of Chemical Bonds
2.2Intramolecular Force and Potential Energy
2.3Ionic Bonding and Ionic Solids
2.4Metallic Bonding and Alloys
2.5Lewis Dot Diagrams
2.6Resonance and Formal Charge
🌀 Unit 3: Intermolecular Forces and Properties
3.0Unit 3 Overview: Intermolecular Forces and Properties
3.2Properties of Solids
3.3Solids, Liquids, and Gases
3.4The Ideal Gas Law
3.5The Kinetic Molecular Theory of Gases
3.6Deviations from the Ideal Gas Law
3.7Mixtures and Solutions
3.8Representations of Solutions
3.9Separation of Solids/Mixtures
3.10Solubility and Solubility Rules
3.11Spectroscopy and the Electromagnetic Spectrum
3.12Quantum Mechanics and the Photoelectric Effect
🧪 Unit 4: Chemical Reactions
4.0Unit 4 Overview: Chemical Reactions
4.1Recognizing Chemical Reactions
4.2Net Ionic Equations
4.4Physical vs. Chemical Changes
4.5Stoichiometry & Calculations
4.6Titrations - Intro and Calculations
4.8Intro to Acid-Base Neutralization Reactions
👟 Unit 5: Kinetics
5.0Unit 5 Overview: Kinetics
5.1Defining Rate of Reaction
5.2Introduction to Rate Laws
5.3Rate and Concentration Change
5.4Writing Rate Laws
5.5Collision Model of Kinetics
5.6Reaction Energy and Graphs w/ Energy
5.7Reaction Mechanisms and Elementary Steps
5.8Writing Rate Laws Using Mechanisms
🔥 Unit 6: Thermodynamics
6.0 Unit 6 Overview: Thermochemistry and Reaction Thermodynamics
6.1Endothermic Processes vs. Exothermic Processes
6.2Energy Diagrams of Reactions
6.3Kinetic Energy, Heat Transfer, and Thermal Equilibrium
6.4Heat Capacity and Coffee-Cup Calorimetry
6.5Phase Changes and Energy
6.6Introduction to Enthalpy of Reaction
6.7Bond Enthalpy and Bond Dissociation Energy
6.8Enthalpies of Formation
⚖️ Unit 7: Equilibrium
🍊 Unit 8: Acids and Bases
8.0Unit 8 Overview: Acids and Bases
8.1Introduction to Acids and Bases
Unit 9: Applications of Thermodynamics
🤺 AP Chemistry Essentials
🧐 Multiple Choice Questions
*ap® and advanced placement® are registered trademarks of the college board, which was not involved in the production of, and does not endorse, this product.
© fiveable 2021 | all rights reserved.
2550 north lake drive
milwaukee, wi 53211