# pH and pKa

Have you ever tasted lemon juice? It's super sour! That's because it's a weak acid. To learn more about weak acids, let's explore the world of pH and pKa.

So, what exactly are pH and pKa? They are measurements that help us understand how acidic or basic a solution is. pH is a number that ranges from 0 to 14, with 0 being the most acidic and 14 being the most basic. pKa is a value that tells us how easily an acid can give away a hydrogen ion.

Now, let's get into the nitty-gritty of how to calculate pH and pKa. We use something called an ICE table to help us out. It sounds complicated, but it's just a table that helps us keep track of the different components in a chemical reaction.

Finally, let's talk about percent ionization. Essentially, it's the percentage of acid molecules that give away a hydrogen ion in a solution. It's an important concept to understand when working with weak acids like lemon juice.

In summary, we've covered the basics of pH and pKa, learned how to calculate them using ICE tables, and explored the concept of percent ionization. Now that you have a better understanding of these concepts, you'll be able to impress your friends with your knowledge of chemistry!

## Understanding the Relationship from pKa to pH

Before diving into pH and pKa, let’s recall the definition of Bronsted-Lowry acids and bases, and also the meaning of conjugate acids and bases.

Bronsted-Lowry acids are proton (H+) donors, whereas Bronsted-Lowry bases are proton (H+) acceptors. Let’s look at the reaction between ammonia and water.

*The reaction between ammonia and water*

Conjugate acids are bases that gained a proton H+. On the other hand, Conjugate bases are acids that lost a proton H+. For example, when HCl is added to H2O, it dissociates to form H3O+ and Cl-. Water will gain a proton, and HCl will lose a proton.

*Conjugate pairs in a reaction between HCl and Water*

In the world of chemistry, sometimes terms can be used interchangeably. For example, H+ and H3O+ both refer to hydrogen ions. Now, let’s dive into the relationship between pH and pKa.

pH is a measurement of the concentration of hydrogen ions in a solution. To learn more about pH, check out our “pH Scale“ article!

To understand pKa, we need to talk about Ka, which is the acid dissociation constant. This measures how well an acid can dissociate in water. Basically, the higher the Ka, the stronger the acid. We use the formula Ka = [H+][A-]/[HA] to calculate it.

pKa is related to Ka, and can be calculated by taking the negative log of Ka, giving us the pKa value. Buffers are solutions that contain a weak acid and its conjugate base, or a weak base and its conjugate acid. The Henderson-Hasselbalch equation helps us understand the relationship between pH, pKa, and the components of a buffer solution. The formula is pH = pKa + log [A-]/[HA].

Now that we’ve covered the basics of pH and pKa, as well as how they relate to each other, we can move on to more advanced calculations and concepts. Stay tuned for more chemistry knowledge!

**Difference between pKa and pH**

The main difference between pH and pKa is that pKa is used to show the strength of an acid. On the other hand, pH value is a measure of the acidity or alkalinity of an aqueous solution. Let’s make a table comparing pH and pKa.

## pH and pKa Equation: Henderson Hasselbalch Equation

When we have a strong acid, such as HCl, it will completely dissociate into H+ and Cl- ions. So, we can assume that the concentration of [H+] ions will be equal to the concentration of HCl.

HCl → H+ + Cl-

However, calculating the pH of weak acids is not as simple as with strong acids. To calculate the pH of weak acids, we need to use ICE charts to determine how many H+ ions we will have at equilibrium, and also use equilibrium expressions (Ka). Using the acid dissociation constant (Ka), we can determine pH by knowing the molar concentration of the acid in the solution.

HA (aq) ⇌ H+ (aq) A- (aq)

Weak acids are those that partially ionize in solution. However, the Henderson-Hasselbalch equation should not be used for extremely low pH acids or concentrated solutions, as it may not provide accurate results in those situations.

**ICE Charts**

The easiest way to learn about ICE tables is by looking at an example. So, let’s use an ICE chart to find the pH of a 0.1 M solution of acetic acid (The Ka value for acetic acid is 1.76 x 10-5).

Step 1: First, write down the generic equation for weak acids:HA (aq) ⇌ H+ (aq) A- (aq)

Actually, pure water does have a little bit of H+ ions (1 x 10-7 M). But, we can ignore it for now since the amount of H+ ions that will be produced by the reaction will be way more significant.

Step 3: Now, we need to fill out the “C” (change) row. When dissociation occurs, change goes to the right. So, the change in HA will be -x, whereas the change in the ions will be +x.

Step 4: The equilibrium row shows the concentration at equilibrium. “E” can be filled by using the values of “I” and “C”. So, HA will have a concentration of 0.1 - x at equilibrium and the ions will have a concentration of x at equilibrium.

*Filling the “E” row on the ICE chart*

Calculating the pH of a weak acid can be tricky, but there is a shortcut that can help you save time on your AP exam. Here’s how to do it:

Step 1: Write the equation for the dissociation of the weak acid. For example, let’s use the dissociation of acetic acid: CH3COOH ⇌ H+ + CH3COO-

Step 2: Write the expression for the acid dissociation constant (Ka). For acetic acid, the expression is Ka = [H+][CH3COO-]/[CH3COOH]

Step 3: Look up the value of Ka for the weak acid you are working with. For acetic acid, Ka = 1.76×10^-5

Step 4: Write down the initial concentration of the weak acid (HA) in the problem. Knowing the weak acid concentration is crucial for setting up the ICE chart and calculating the pH. For example, if we have a 0.1 M solution of acetic acid, then [HA] = 0.1 M.

Step 5: Use the equation [H+] = Ka × initial concentration of HA to find the concentration of hydrogen ions in the solution. For acetic acid, [H+] = (1.76×10^-5) (0.1) = 0.00176 M.

Step 6: Use the concentration of hydrogen ions to calculate the pH of the solution. pH = -log10 [0.00176] = 2.75.

And there you have it! With this shortcut, you can quickly and easily calculate the pH of a weak acid without having to construct an ICE table. Just remember to memorize the equation [H+] = Ka × initial concentration of HA, and you’ll be ready to tackle any pH problem that comes your way.

**pH and pKa Formulas**

Step 3: Use the Ka formula to set up an equation for the equilibrium constant.

Ka = [H+][A-]/[HA] = (5.0×10^-6)^2 / (0.010 - 5.0×10^-6)

Step 4: Solve for Ka.

Ka = 1.25×10^-10

Step 5: Use the pKa formula to find pKa.

pKa = -log10 (Ka) = -log10 (1.25×10^-10) = 9.9

Therefore, the pKa values of weak acids can be determined using these calculations, and in this case, the pKa value is 9.9.

Remember, understanding pH and pKa is essential in chemistry, especially in acid-base reactions. With these formulas and examples, you can now solve problems related to pH and pKa with ease!

*ICE chart for a 0.010 M solution of a weak acid*

Step 3: Write the equilibrium expression using the values in the equilibrium row (E), and then solve for Ka.

Ka = [products][reactants]= [H+][A-]HA = X20.010 - XKa = (5.0×10-6)(5.0×10-6)0.010 - 5.0×10-6 = 2.5×10-9 mol dm-3

Step 4: Use the calculated Ka to find pKa.

pKa = -log10 Ka = -log10 (2.5×10-9)pKa = 8.6

## Finding Percent Ionization of a Weak Acid given pH and pKa

Another way of measuring the strength of acids is through percent ionization. The formula to calculate percent ionization is given as:

% ionization = Concentration of H+ ions in equilibriumInitial concentration of the weak acid = x[HA] × 100

The acid ionization constant (Ka) is used in these calculations to determine the extent of ionization of the acid in solution.

Remember: the stronger the acid, the greater the % ionization. Let’s go ahead and apply this formula to an example!

Find the Ka value and the percent ionization of a 0.1 M solution of a weak acid containing a pH of 3. 1. Use the pH to find [H+]. [H+] =10-pH[H+] =10-3 2. Make an ICE table to find the concentrations of HA, H+, and A- in equilibrium.

*ICE table of a 0.1 M solution of a weak acid*

The question of how to calculate pH and pKa of weak acids, what are the formulas relating pH and pKa, how to calculate pH and pKa from Ka and concentration, and how to calculate pKa from pH and concentration can be answered by looking at the information provided in the two texts above. The texts provide information about Chapter 9 - Molecular Geometry and Bonding and Chad the Drake of Gen Chem, both of which are related to the topic of weak acids.

To answer the question, one can look at the information provided in Chapter 9 - Molecular Geometry and Bonding. This chapter provides an overview of the concepts related to weak acids, including the formulas for calculating pH and pKa. Specifically, the Henderson-Hasselbalch equation can be used to calculate the pH and pKa of a weak acid, given the Ka and concentration of the acid. The equation is as follows:

pH = pKa + log([A-]/[HA])

The equation can also be used to calculate pKa from pH and concentration, by rearranging the equation to solve for pKa.

pKa = pH - log([A-]/[HA])

## pH and pKa: Acid Dissociation Constant

**How to calculate pH from pKa and concentration?**

To calculate the pH and pKa of weak acids, we need to use an equilibrium expression and an ICE chart. For example, carbonic acid in aqueous solution donates a proton to become H+ and bicarbonate, illustrating the behavior of acids in solution.

**Are pH and pKa the same?**

No, they are not the same. pH is a measurement of the [H+] ion concentration in a solution. For instance, hydrochloric acid is a strong acid with a very low pKa value, demonstrating its high acidity and complete dissociation in solution. On the other hand, pKa is used to show whether an acid is strong or weak.

**How are pH and pKa related?**

In buffers, pH and pKa are related through the Henderson-Hasselbalch equation.

**What is pKa and pH?**

pH is the negative log (base 10) of [H+]. pKa is the negative log (base) of Ka.