Ka and Kb - Online Tutor, Practice Problems & Exam Prep

• KA and KB are equilibrium constants for measuring acid and base strengths of weak acids and bases
• KA represents acid dissociation constant while KB represents base association constant
• Stronger acids have higher KA values while weak acids have KA values less than 1
• Stronger bases have higher KB values while weak bases have KB values less than 1
• KA and KB are related to ionization constant of water KW
• KW = KA x KB
• PKA and PKB are negative logs of KA and KB respectively
• Strong acids have PKA values less than 1 and weak acids have PKA values greater than 0
• Strong bases have PKB values less than 1 and weak bases have PKB values greater than 0
• KA and KB can be used to convert between weak acid and its conjugate base form

Characteristics of Ka and Kb

K and K_B are equilibrium constants for acid and bases respectively, and they are used to measure the strengths of acids of weak acids and bases. Now if we take a look here, we can talk about the different types of equilibrium constants in reference to K_A and K_B. We can then show great examples of how they relate to typical types of acid base reactions. And then finally we can talk about the relationships they have between their strong and weak forms.

Now here we're going to say that K_A represents our acid dissociation or ionization constant. A great example here is hydrofluoric acid. It is a weak binary acid. Since it is an acid, it donates H by donating H⁺. It becomes F^- and water gaining the H acting as the base becomes H₃O⁺. Here K_A is an equilibrium constant and like other equilibrium constants it has a ratio of products over reactants. But remember we ignore solids and liquids here. Water is a liquid so it would not be included in our equilibrium expression.

Here K would equal the concentration of fluoride ions times the concentration of hydronium ions divided by the concentration of hydrofluoric acid. This expression will be equal to the value of K_A, the K_A of our weak acid. This value happens to be 6.3×10-4. Now what can we talk about in terms of acid base strengths in relation to K_A? While we can say here, the stronger the acid, the higher the K_A value will be, We can say here that weak acids tend to have K_A values less than one and strong acids tend to have K values greater than one.

Now that we've talked about the K_A value, let's look at K_B. K_B is the base association or ionization constant. It is used for weak basis. Here. A week basis that we see is the ammonium molecule or ammonia molecule. It is the base. So water acts as the acid to donate an H⁺ away. Ammonia accepts an H⁺ and becomes the ammonium ion because water loss in H⁺ it becomes the hydroxide ion K_B. Just like K_A is an equilibrium constant, so it is a ratio of products over reactants. Again, we still ignore solids and liquids.

So here it would be equal to the hydronium ion concentration times hydroxide ion concentration divided by the ammonia concentration. Here the K_B value of ammonia is equal to 18×10-5. Here, the stronger the base is, then the higher its K_B value will be. We say here that weak basis tend to have K_B values less than one and strong basis tend to have them greater than one.

Now here strong acids and bases have a dissociation constant associated with them as well. It's just that for strong acids their K_A values are greater than one, so greater than one that we don't need to talk about them. Strong bases have K_B values much greater than one, so again we don't talk about their K_B values either. K_A, we mainly stick with weak acids, K_B we stick with weak basis, right? So just remember this, these are just association constants related to the weak forms of acid and bases.

Ka and Kb Example

Identify the strongest acid from the following list of weak acids based on their K_a values. Here assume temp is 25℃. Now here we're saying that because K_a is just like other equilibrium constants, it's affected by changes in temperature. Here we're saying 25℃ because this is seen as standard temperature or room temperature, where we have a good fix on what the K_a value for a weak acid would be.

If we take a look here, we have four different weak acids with K_a values. We know all of them are weak because they have K_a values less than one. We want the strongest acid and remember higher your K_a value, the stronger the answer will be and we take a look here. This is to the -10, -14, -4 and -4. So these two are out. So it's either C or D here.

This is 4.6 × 10^-4. This one's only 1.4 × 10^-4 options sees the correct answer. Nitrous acid has a K_a value of 4.6 × 10^-4 and would represent the strongest weak acid from the options given below.

Hypobromous acid (K_a = 2.8 × 10⁻⁹) and hydrocyanic acid (K_a = 4.9 × 10⁻¹⁰) are both weak acids. Determine if reactants or products are favored in the following reaction.

HBrO (aq) + CN⁻ (aq) ⇌ BrO⁻ (aq) + HCN (aq)

a) reactants        b) products            c) both directions are favored equally   d) neither direction is favored

• A. reactants
• B. products
• C. both directions are favored equally
• D. neither direction is favored

Identify a Bronsted-Lowry acid with weakest conjugate base.

a) H₃BO₃  K_a = 5.4 × 10⁻¹⁰

b) HF K_a = 3.5 × 10⁻⁴

c) HNO₂  K_a = 4.6 × 10⁻⁴

d) HClO K_a = 2.9 × 10⁻⁸

• A. H₃BO₃ K_a = 5.4 × 10⁻¹⁰
• B. HF K_a = 3.5 × 10⁻⁴
• C. HNO₂ K_a = 4.6 × 10⁻⁴
• D. HClO K_a = 2.9 × 10⁻⁸

Ka and Kb Relationship

KA and KB are related to the following formulas and can only be used for conjugate pairs. Here we're going to say that kW, which remember is our ionization constant for water, equals KA times KB. When we take the negative log of kW, KA and KB then it becomes 14=pKA+pKB.

Now what exactly do I mean by conjugate pairs? Well, here we have nitrous acid which has a KA value of 4.6×10-4. Its conjugate base is the nitrite ion. We can use this formula here to convert KA of the weak acid form to KB of the weak basic form. So that's what we mean. The weak acid form in its conjugate base form, they're connected to each other by KA and KB and their product equals kW.

Now here when we talk further about pKA and pKB, we can say for acids we have pKA. Remember p just stands for negative log. So pKA means negative log of KA. If we know the pKA, we can find KA as well because here we can say that KA=10-pKA. These formulas allow us to go between pKA and KA at whim.

Now here let's talk about strength. Well here we can say that the stronger the acid, the higher the KA and the higher the KA the lower the pKA. A strong acid tends to have a KA greater than one and a pKA less than one. A weak acid tends to have a KA less than one and a pKA value greater than 0.

For bases here, we're going to say that pKB just means that negative log of KB. If we know pKB, then we know KB because KB=10-pKB. In terms of strength, we can say the stronger the base then the higher the KB and the lower the pKA. Strong bases tend to have KB greater than one and pKA less than one. Weak acids tend to have KB values less than one and pKB values greater than 0, right?

So just remember we have these connections between our acid association constant KA and the base association constant KB. We can interchange between them when given the weak acid and its conjugate base. We can also establish relationships in terms of strengths of acids when comparing KA, KB, pKA and pKB.

Ka and Kb Example

Here we're going to say that aspirin, also known as acetyl salicylic acid, has a K value of 3.3×10-4 and it's a medication used to reduce pain, fever and inflammation. Now here it says accompanied the KB value of its conjugate base form. Now here we know it's a conjugate base form because the names are very similar. The difference is this one ends with ick acid. I'm denoting its acid form and this ends with eight which represents its conjugate base form.

So we have a weak acid which has its K value and we're looking for the KB of its conjugate base form. In this case we'd say kW=KA×KB. Here we're assuming the reaction is happening at 25°C since temperature is not given to us. So kW=1.0×10-14, KA is 3.3×10-4. We're looking for KB. So divide both sides here by 3.3×10-4. Those cancel out.

When you unch it into your calculator, you'll get KB equals 3.0×10-11. So this would represent our base association constant for the conjugate base form of aspirin.

Identify which of the compounds is the strongest species.

a) Iodic acid pK_a = 0.80  b) Acetic acid pK_b = 9.24 c) Formic acid pK_a = 3.75 d) Ammonium pK_b = 4.75 

• A. Iodic acid pK_a = 0.80
• B. Acetic acid pK_b = 9.24
• C. Formic acid pK_a = 3.75
• D. Ammonium pK_b = 4.75

Determine the pK_a given the Kb of the following bases:

i) NH₃               K_b = 1.76 × 10⁻⁵ ; NH₄⁺           pK_a = ____________

ii) C₆H₅NH₂     K_b = 3.9 × 10⁻¹⁰ ; C₆H₅NH₃⁺  pK_a = ____________

• A. i. pK_a = 18.754, ii. pK_a = 23.41
• B. i. pK_a = 9.246, ii. pK_a = 4.59
• C. i. pK_a = 4.754, ii. pK_a = 9.41
• D. i. pK_a = −9.246, ii. pK_a = −4.59