How do you determine pKa from structure?

The information here is to help you decide which structure of an acid or base will dominate at a particular pH. Let’s do a general case. pKa is defined as -log10 Ka where Ka = [H+][A-] / [HA]. This tells us that when the pH = pKa then log [HA] / [A-] = 0 therefore [HA] = [A-] ie equal amounts of the two forms.

How do you tell if a molecule has a high pKa?

A pKa may be a small, negative number, such as -3 or -5. It may be a larger, positive number, such as 30 or 50. The lower the pKa of a Bronsted acid, the more easily it gives up its proton. The higher the pKa of a Bronsted acid, the more tightly the proton is held, and the less easily the proton is given up.

How does molecular structure affect pKa?

The lower the pKa, the stronger the acid. Several structural elements of a molecule can affect pKa including: Resonance – If a molecule has electrons that are spread over more than one bond (if a molecule has resonance), it will have a lower pKa compared to a molecule that does not.

How can we determine pKa of a molecule using potentiometric titration?

In a potentiometric titration, a sample is titrated with acid or base using a pH electrode to monitor the course of titration. The pKa value is calculated from the change in shape of the titration curve compared with that of blank titration without a sample present.

Is Chem3D free?

An alternative for Mac users As you cannot download Chem3D to a Mac, this free software from ChemAxon may be a useful alternative: MarvinSketch.

What is the formula for PKA?

Sodium bicarbonate has a pKa of 6.4 or 10.32, depending on the type of reaction. The pKa is derived from the acid dissociation constant, Ka, through the formula pKa = -log(Ka).

How to calculate Ka from pKa.?

Ka = [H+] [A-]/ [HA] pKa = – log Ka at half the equivalence point, pH = pKa = -log Ka

How do you calculate pKa from pH?

Referring to the definitions of pH and pKa, this becomes: -pKa = -pH + log [A-]/[HA] Finally, after adding pH and pKa to both sides: pH = pKa + log [A-]/[HA]. This equation allows you to calculate pH if the dissociation constant, pKa, and the concentrations of the acid and conjugate base are known.

How to calculate pKa from pH?

If you know either pH or pKa , you can solve for the other value using an approximation called the Henderson-Hasselbalch equation: pH = pKa + log ([conjugate base]/ [weak acid]) pH = pka+log ([A – ]/ [HA]) pH is the sum of the pKa value and the log of the concentration of the conjugate base divided by the concentration of the weak acid.