Question

why we convert pka to -log ka​

Answer 1

he chemical nature of any substance depends on the charged particles that it produces, called 'ions'. By this principle, an acid is a substance which readily donates its hydrogen ions (called protons) when dissolved in water. On the other hand, a base is a substance which readily accepts such free protons. Based on how eagerly these substances donate or accept such protons, they are categorized as 'strong' or 'weak'. This is how the Brønsted-Lowry Theory describes the concept of an acid and a base.

While pH is a popular value used to indicate the number of protons that are produced by a substance, there are other quantities, like Ka, Kb, pKa, and pKb, used to explain the strength of these substances. They are commonly used by chemists to decide the strength of chemical reactants in laboratories, and by pharmacists to understand the dosage of drugs. However, these values can only be used for Brønsted acids and bases, i.e., those substances which give or accept protons. This is because, other theories, like the Lewis Theory, define acids and bases in terms of electron transfers, so values like Ka and pKa, which deal with protons, lose their relevance. The following sections describe the conversion of the pKa value of an acid to its Ka value.

Answer 2

When citing the strength of an acid, chemists often use the dissociation constant, Ka, but this number can vary by several orders of magnitude from one acid to another. To create a more manageable number, chemists define the pKa value as the negative logarithm of the Ka value: pKa = -log Ka. If you already know the pKa value for an acid and you need the Ka value, you find it by taking the antilog. In practice, this means raising both sides of the equality to exponents of 10.


Converting From pKa to Ka

In some tables, you may find the pKa value listed, but you may need the Ka value to plug into your equations. The mathematical operation you perform is Ka = antilog (-pKa). You solve this by raising both sides of the original relationship to powers of 10 to get:

Ka = 10(-pKa)

When pKa is a whole number, such as -7, this operation is easy to perform, but when it contains a fraction, such as 7.5, you may have to look up the value in a table. You can also find it on a scientific calculator by inputting the number and pressing the exponent key, which either looks like a hat (^) or is denoted by 10x. Remember that pKa is expressed as a common logarithm (base 10) and not as a natural logarithm (base e), so you want to find a table or select a function on your calculator that raises the number to a power of 10 rather than a power of e.