Acids & Bases
The Exponential Nature:
An Example:
The pH of sea water is 8.5. pH is expressed by the formula pH = -log [H+] where [H+] is the hydrogen ion concentration in moles per liter. Find the [H+] value for sea water.
The Solution:
We know that 8.5 = -log [H+]. To find [H+], we rewrite: [H+] = 10-8.5. Using a calculator to evaluate 10-8.5, we get [H+] = 3.16 x 10-9 M.
Thanks for visiting! Hope this helped clear up any misinformation you had about Acids&Bases.
Big thanks to our chemistry teacher Mr.Cork and John T. Moore, EdD for writing Chemistry for Dummies, for which without, we wouldn't have been able to create this webpage.
Putting Coffee and Other Substances on the pH Scale:
The amount of acidity in a solution is related to the concentration of the hydronium ion in the solution. The more acidic the solution is, the larger the concentration of the hydronium ion. In other words, a solution in which the [H3O+] = 1.0x10^-2 is more acidic than a solution in which the [H3O+]= 1.0x10^-7. The pH scale, a scale based on the [H3O+], was developed to more easily tell at a glance the relative acidity of a solution. pH is defined as the negative logarithm (abbreviated as log) of the [H3O+]. Mathematically, it looks like this:
pH = -log [H3O+]
Based on the water dissociation constant, Kw (see Playing Both Parts: Amphoteric Water), in pure water the [H3O+] equals 1.0x10^-7. Using this mathematical relationship, you can calculate the pH of pure water:
pH = -log [H3O+]
pH = -log [1.0x10^-7]
pH = -[-7]
pH = 7
The pH of pure water is 7. This point on the pH scale is called neutral.
A solution is called acidic if it has a larger [H3O+] than water and a smaller pH value than 7.
A basic solution has a smaller [H3O+] than water and a larger pH value than 7.
The pH scale really has no end. You can have a solution of pH that registers less than 0. ( A 10 M HCl solution, for example, has a pH of -1.) However, the 0 - 14 range is a convenient range to use for weak acids and bases and for dilute solutions of strong acids and bases.
Fun Fact:
The approximate pH of a pinapple is
3.20 -4.00
For more facts of the ph of food, visit: @http://www.eidusa.com/Theory_pH_FOOD.htm!
The [H3O+] of a 2.0 M acetic acid solution is 6.0x10^-3. Looking at the pH scale, you can see that this solution is acidic. Now calculate the pH of this solution:
pH = -log [H3O+]
pH = -log [6.0 x 10^-3]
pH = -[-2.22]
pH = 2.22
Playing Both Parts; Amphoteric Water:
Water can act as either an acid or a base, depending on what it's combined with. When an acid reacts with water, water acts as a base, or a proton acceptor. But in reactions with a base (like ammonia), water acts as an acid, or a proton donor. Substances that can act as either an acid or a base are called amphoteric.
But can water react with itself? Yes, it can. When two molecules react with each other, one donates a proton and the other accpets it:
H2O(L) + H2O(L) <=> H3O+(aq) + OH-(aq)
This reaction is an equilibrium reaction. A modified equilibrium constant, called the Kw (water dissociation constant), is associated with this reaction. The Kw has a value of 1.0x10^-14 and has the following form:
1.0x10^-14= Kw =[H3O+][OH-]
In pure water, the [H3O+] equals the [OH-] from the balanced equation, so [H3O] = [OH-] = 1.0x10^-7. The Kw value is constant. The value allows you to convert from [H+] to [OH-], and vice versa, in any aqueous solution, not just pure water.
In aqueous solutions, the hydronium ion and hydroxide ion concentrations are rarely going to be equal. But if you know one of them, the Kw value allows you to figure out the other one.
Take a look at the 2.0 M acetic acid solution problem under "Ionization Partway; Weak Acids." You find that the [H3O+] is 6.0x10^-3. Now you have a way to calculate the [OH-] in the solution by using the Kw relationship:
Kw = 1.0x10^-14 = [H3O+][OH-]
1.0x10^-14 = [6.0x10^-3] [OH-]
1.0x10^-14/6.0x10^-3 = [OH-]
1.7x10^-12 = [OH-]
In the section "Playing Both Parts: Amphoteric Water," I explain that the Kw expression enables you to calculate the [H3O+] if you have the [OH-].
Another equation called the pOH is the negative logarithm of the [OH-]. You can calculate the pOH of a solution just like the pH by taking the negative log of the hydroxide ion concentration. If you use the Kw expression and take the negative log of both sides, you get 14 = pH+pOH. This equation makes going from pOH to pH quite easy.
Just as you can convert from [H3O+] to pH, you can aslo go from pH to [H3O+]. To do this, you can use what's called the antilog relationship, which is
[H3O+] = 10^-pH
Human blood has a pH of 7.3. To calulate the [H3O+] from the pH of blood:
[H3O+] = 10^-pH
[H3O+] = 10^-7.3
[H3O+] = 5.01x10^-8
The same prodecure can be used to calculate the [OH-] from the pOH.
To sustain life, human blood must stay withim about +/-0.2 pH units of 7.3, a narrow range. Many things in our environment, such as foods and hyperventilation (breathing too fast), can act to change the pH of our blood. Buffers help to regulate blood pH and keep it in the 7.1 - 7.5 range.
Hey, Litmus Paper, why so blue?
Identifying Acids and Bases with Indicators:
Indicators are substances (organic dyes) that change color in the presence of and acid or base. You may br familiar with an acid-base indicator plant -- the hydrangea*. If it's grown in acidic soil, it turns pink; if it's grown in alkaline (basic) soil, it turns blue. Another common substance that acts as a good acid-base indicator is red cabbage. Mr. Cork will have his students chop some up and boil it (we all really love that part). Then use the leftover liquid to test substances. When mixed with acid, the liquid turns pink; when mixed with a base, it turns green. In fact, if you take some of this liquid, make it slightly basic, and then exhale your breath into it through a straw, the solution eventually turns pink, indicating that the solution has turned slightly acidic. The carbon dioxide in your breath reactcs with the water, forming carbonic acid. This reaction explains why rainwater is slightly acidic, it reacts with the carbon dioxide from the atmosphere as it falls to earth.
The two most commonly used indicators are Litmus paper and Phenolphthalein.
*
Litmus is a substance that is extracted from a type of lichen and absorbed into porous paper. (In case you are scheduled for a hot game of Trivial Pursuit this weekend, lichen is a plant that's made up of an alga and a fungus that live intimately together and mutually benefit from the relationship. Sounds kind of sordid to me.)
The Three different types of litmus:
-
Red litmus is used to test bases.
-
Blue litmus is sued to test acids.
-
Neutral can be used for both.
If a solution is acidic, both blue and neutral litmus turn red. If a solution is basic, both red and neutral litmus will turn blue.
Litmus paper is a good, quick test for acids and bases. And you don't have to put up with the smell of boiling cabbage.
Titrating with Phenolphthalein (pronounced fe-nul-tha-leen):
Phenolphthalein is another commonly used indicator. Until a few years ago, phenolphthalein was used as the active ingredient in a popular laxative. One could extract the phenolphthalein from the laxative by soaking it in rubbing alcohol or gin.
Phenolphthalein is clear and colorless in an acid solution and pink in a basic solution. It's commonly used in a procedure called a titration, where the concentration of an acid or base is determined by its reaction with a base or acid of known concentration.