What scientific concept do you need to know in order to solve this problem? Our tutors have indicated that to solve this problem you will need to apply the Equilibrium Expressions concept. If you need more Equilibrium Expressions practice, you can also practice Equilibrium Expressions practice problems. Our tutors rated the difficulty of Methylamine is a weak base in water according to the equilib Our expert Chemistry tutor, Jules took undefined 45 seconds to solve this problem.
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Hydroxide ion or acetate ion? Let's put our understanding of the pK a concept to use in the context of a more complex molecule. For example, what is the pK a of the compound below? We need to evaluate the potential acidity of four different types of protons on the molecule, and find the most acidic one. The aromatic protons are not all acidic - their pK a is about The amine group is also not acidic, its pK a is about Remember, uncharged amines are basic: it is positively-charged protonated amines, with pKa values around 10, that are weakly acidic.
The alcohol proton has a pK a of about 15, and the phenol proton has a pK a of about thus, the most acidic group on the molecule above is the phenol. Be sure that you can recognize the difference between a phenol and an alcohol - remember, in a phenol the OH group is bound directly to the aromatic ring.
If this molecule were to react with one molar equivalent of a strong base such as sodium hydroxide, it is the phenol proton which would be donated to form a phenolate anion. Identify the most acidic functional group on each of the molecules below, and give its approximate pK a. By definition, the pK a value tells us the extent to which an acid will react with water as the base, but by extension we can also calculate the equilibrium constant for a reaction between any acid-base pair.
Mathematically, it can be shown that:. The first step is to identify the acid species on either side of the equation, and look up or estimate their pK a values. On the left side, the acid is of course acetic acid while on the right side the acid is methyl ammonium ion in other words, methyl ammonium ion is the acid in the reaction going from right to left. We can look up the precise pK a values in table 7 at the back of the book , but we already know because we have this information memorized, right?!
More precise values are 4. Without performing any calculations at all, you should be able to see that this equilibrium lies far to the right-hand side: acetic acid has a lower pK a , meaning it is a stronger acid than methyl ammonium, and thus it wants to give up its proton more than methyl ammonium does. Doing the math, we see that. So K eq is a very large number much greater than 1 and the equilibrium for the reaction between acetic acid and methylamine lies far to the right-hand side of the equation, just as we had predicted.
This also tells us that the reaction has a negative Gibbs free energy change, and is thermodynamically favorable. Show the products of the following acid-base reactions, and roughly estimate the value of K eq. Recall from your General Chemistry course that a buffer is a solution of a weak acid and its conjugate base. The key equation for working with buffers is the Henderson-Hasselbalch equation:. The equation tells us that if our buffer is an equimolar solution of a weak acid and its conjugate base, the pH of the buffer will equal the pK a of the acid because the log of 1 is equal to zero.
If there is more of the acid form than the base, then of course the pH of the buffer is lower than the pK a of the acid. What is the pH of an aqueous buffer solution that is 30 mM in acetic acid and 40 mM in sodium acetate?
The pK a of acetic acid is 4. The Henderson-Hasselbalch equation is particularly useful when we want to think about the protonation state of different biomolecule functional groups in a pH 7 buffer.
When we do this, we are always assuming that the concentration of the biomolecule is small compared to the concentration of the buffer components. The actual composition of physiological buffer is complex, but it is primarily based on phosphoric and carbonic acids. Imagine an aspartic acid residue located on the surface of a protein in a human cell. Being on the surface, the side chain is in full contact with the pH 7 buffer surrounding the protein.
In what state is the side chain functional group: the protonated state a carboxylic acid or the deprotonated state a carboxylate ion? Doing the math, we find that the ratio of carboxylate to carboxylic acid is about to 1: the carboxylic acid is almost completely ionized in the deprotonated state inside the cell.
This result extends to all other carboxylic acid groups you might find on natural biomolecules or drug molecules: in the physiological environment, carboxylic acids are almost completely deprotonated.
We can do the same rough calculation for other common functional groups found in biomolecules. It will be very helpful going forward to commit the following to memory:.
We will talk about the physiological protonation state of phosphate groups in chapter 9. The molecule below is not drawn in a reasonable protonation state for pH 7. Redraw it in with the functional groups in the correct protonation state.
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