To create a more manageable number, chemists define the pKa value as the negative logarithm of the Ka value: pKa = -log Ka. For example, log(1000) = 3, because 10^3 = 1000. In practice, this means raising both sides of the equality to exponents of 10. 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. Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan. If the number of ions exceed that of the acid molecules, then the acid is strong, since it easily splits up into ions (dissociates) . Thus, another formula to convert pKa into Ka is to find the value of 10 raised to the negative pKa value. Fractional Distillation Vs. Some not-so-acidic compounds. Purdue University: Brønsted Acids and Bases, Khan Academy Organic Chemistry: Ka and pKa Derivation. What are simple machines? Based on how eagerly these substances donate or accept such protons, they are categorized as ‘strong’ or ‘weak’. Thus, the logarithm of 1000 is 3. pKa = –log Ka. On the other hand, a base is a substance which readily accepts such free protons. This article highlights the topic of simple…. If the value of Ka is high, then the acid is strong, since the numerator (concentration of ions) in the equation is high. pOH = -log [OH-] pKa = -log Ka. 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. These cookies do not store any personal information. Let ‘HA’ be a molecule of an acid in an aqueous solution.Then it dissociates into ions, as given below. It is clear how the Ka value can be calculated in a single step, if the pKa value is available. For example, in case of the number 1000, 10 must have the exponent 3 to obtain 1000, i.e., 103 = 1000. This ScienceStruck post tells you how to convert pKa to Ka, with the help of an equation relating the two values. Whether the acid is a strong or weak one depends on the proportion of these molecules and ions. They are commonly used by chemists to decide the strength of chemical reactants in laboratories, and by pharmacists to understand the dosage of drugs. When an acid is dissolved in water, its molecules split up into different ions (atoms with positive or negative charges). K °C °F; Pressure converter. Thus, strong acids have a high Ka and a low pKa, while weak acids have a low Ka values and a high pKa. You also have the option to opt-out of these cookies. Despite this, there is a lot of confusion between the two concepts. HCN Ka=5.8 x 10^-10 pKa = 9.24. pH to pKa. From the above equation it is clear that the higher the Ka value, the lower pKa will be. p is equal to “negative log ”. Convert. HA (acid) ⇋ H + (proton) + A – (conjugate base) The value of K a for this acid is given by the following equation. The pKa measures the "strength" of a Bronsted acid. Deriving Ka from pH. A large Ka value also means the formation of products in the reaction is favored. K a = [H + ] [A –] ÷ [HA] If the value of K a is high, then the acid is strong, since the numerator (concentration of ions) in the equation is high. The logarithm of the number in parentheses -- Ka, in this case -- is the power or exponent to which the number 10 is raised to equal that number. Equations for converting between Ka and Kb, and converting between pKa and pKb. Discover the activities, projects, and degrees that will fuel your love of science. In other words, a low pKa indicates a strong acid and vice versa. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts. Convert. For example, the Ka constant for acetic acid (CH 3 COOH) is 0.0000158 (= 10-4.8), but the pKa constant is 4.8, which is a simpler expression. at half the equivalence point, pH = pKa = -log Ka. Figure AB9.4. On the other hand, if we are given only the logarithmic value, i.e., 3, then, to find the value of the number from which this logarithm was obtained, we have to take the antilogarithm of this logarithmic value. The chemical nature of any substance depends on the charged particles that it produces, called ‘ions’. The lower the pKa, the stronger the acid and the greater the ability to donate a proton in aqueous solution. The dissociation constant for a strong acid can be as high as 107, while for a weak acid it can be as low as 10-12. Relationship between Ka of a weak acid and Kb for its conjugate base. i.e., the antilogarithm of the exponent (logarithm) gives the value of 10 raised to that exponent, i.e., the value of 10 raised to the logarithm itself. He began writing online in 2010, offering information in scientific, cultural and practical topics.