Ka = x2/(0.9 - x) As noted above, [H3O+] = 10-pH. Since x = [H3O+] and you know the pH of the solution, you can write x = 10-2.4. It is now possible to find a numerical value for Ka. Ka = (10-2.4)2 /(0.9 - 10-2.4) = 1.8 x 10-5.
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.
The pKa is the pH value at which a chemical species will accept or donate a proton. The lower the pKa, the stronger the acid and the greater the ability to donate a proton in aqueous solution. The Henderson-Hasselbalch equation relates pKa and pH.
| Ka | Acid |
|---|
| Large | Perchloric acid | HClO4 |
| 3.2 * 109 | Hydroiodic acid | HI |
| 1.0 * 109 | Hydrobromic acid | HBr |
| 1.3 * 106 | Hydrochloric acid | HCl |
Strong acids completely dissociate in aq solution (Ka > 1, pKa < 1).
Buffers are solutions that contain a weak acid and its a conjugate base; as such, they can absorb excess H+ions or OH– ions, thereby maintaining an overall steady pH in the solution. pH is equal to the negative logarithm of the concentration of H+ ions in solution: pH = – log[H+].
Strong AcidsGenerally, a strong acid has a pH of about zero to 3. However, because pH measures the amount of hydrogen ions released in a solution, even a very strong acid can have a high pH reading if its concentration is very dilute.
The smaller the number the weaker it is. If Ka > 0 it is a strong acid, if Ka < 0 it is a weak acid.
The greater the value of Ka, the more favored the H+ formation, which makes the solution more acidic; therefore, a high Ka value indicates a lower pH for a solution. The Ka of weak acids varies between 1.8×10−16 and 55.5. Acids with a Ka less than 1.8×10−16 are weaker acids than water.
The higher Ka is, the more easily the acid dissociates, and the stronger it is (i.e. the weaker the base it is, and the less strongly its bonds are held together by electron donation).
For an aqueous solution of a weak acid, the dissociation constant is called the acid ionization constant (Ka). Similarly, the equilibrium constant for the reaction of a weak base with water is the base ionization constant (Kb).
Weak acids and bases are indicators. All indicators are weak acids. Any indicator changes color when the pH of its solution is 7.
Ka is acid dissociation constant and represents the strength of the acid. pKa is the -log of Ka, having a smaller comparable values for analysis. They have an inverse relationship. Larger the Ka, smaller the pKa and stronger the acid.
To calculate the pH of an aqueous solution you need to know the concentration of the hydronium ion in moles per liter (molarity). The pH is then calculated using the expression: pH = - log [H3O+]. On a calculator, calculate 10-8.34, or "inverse" log ( - 8.34).
The procedure for calculating the pH of a solution of a weak base is similar to that of the weak acid in the sample problem. However, the variable x will represent the concentration of the hydroxide ion. The pH is found by taking the negative logarithm to get the pOH, followed by subtracting from 14 to get the pH.
Key Concepts
- The hydrogen ion concentration in a solution, [H+], in mol L-1, can be calculated if the pH of the solution is known.
- pH is defined as the negative logarithm (to base 10) of the hydrogen ion concentration in mol L-1pH = -log10[H+]
- [H+] in mol L-1 can be calculated using the equation (formula): [H+] = 10-pH
know that Kw = [H+][OH- ] be able to calculate the pH of a strong base from its concentration.
To find the pOH, simply subtract the pH from 14. In order to calculate the pOH, take the negative log of the hydroxide ion concentration. To find the pH, simply subtract pOH from 14.
If the pH is higher than the pKa, then the compound will be deprotonated. A further consideration is the charge on the compound. Acids are neutral when protonated and negatively charged (ionized) when deprotonated. Bases are neutral when deprotonated and positively charged (ionized) when protonated.
pKa<3 is for a strong acid. 3<pKa<7 is for a weak acid. 7<pKa<11 is for a weak base. pKa>11 is for a strong base.
Therefore, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows. In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3.8, so that means lactic acid is a stronger acid than acetic acid.
The value of the buffer capacity is strongly related to the concentrations of ingredients used and increases with their increase. Buffer solutions with a pH equal to the pKa value of the acid (used to make this solution) have the greatest buffering capacity.
At what volume of added base does pH=pKa? At what volume of added acid does pH=14−pKb? It is at the half-equivalence point when pH=pKa, where pKa=14−pKb.
The Key Rule Of Acid-Base Reactions: Stronger Acid Plus A Stronger Base Produces A Weaker Acid and A Weaker Base. Where do we start with this problem? Remember that a pKa table ranks molecules in order of their acidity, from strongly acidic (e.g. HCl with pKa of –8) to weakly acidic (e.g. methane, pKa of ~50).
The higher the pKa of a Bronsted acid, the more tightly the proton is held, and the less easily the proton is given up. The pKa scale as an index of proton availability. Low pKa means a proton is not held tightly. pKa can sometimes be so low that it is a negative number! High pKa means a proton is held tightly.
