BIOL 1406

PreLab 3.1

What is dissociation?

A Note about Scientific Notation

Scientists generally use a system referred to as “Scientific Notation” to express very large or very small numbers.

For example, using scientific notation, the number 123,000,000,000 would be written: 1.23 x 1011. In this case, the positive exponent 11 indicates that we would take the number 1.23 and move the decimal point 11 places to the RIGHT to get the original number.

On the other hand, the number 0.000000123 would be written: 1.23 x 10-7. In this case, the negative exponent -7 indicates that we would take the number 1.23 and move the decimal point 7 places to the LEFT to get the original number.

Most calculators and computers present very large and very small numbers in scientific notation. Because superscripted exponents like 10-7 cannot always be represented on video or electronic displays, several alternative methods for expressing scientific notation have been developed. For example, 1.23 x 10-7 can be written in any of the following 3 ways:

  • 1.23E-7 where “E” means “10 raised to the power of”

  • 1.23e-7 where “e” means “10 raised to the power of”

  • 1.23x10^-7 where “^” means “raised to the power of”

    • Example: 1.23 x 107 would be written as 1.23x10^7 -- Note that when this notation is used ALL spaces are omitted.

    • Example: 2.34 x 105 cm3 would be written as 2.34x10^5cm^3 -- Note that when this notation is used ALL spaces are omitted.

Because it is not possible to enter superscripted exponents (e.g. 10-7 ) in the answer boxes that appear in this Lab Manual and in the Blackboard Quizzes, you must enter answers with exponents using the last method shown, i.e. using the symbol  "^" to mean "raised to the power of." This method will also be used when hints or answers are shown in the pop-up boxes.

Special Note: On all Blackboard quizzes, when you enter answers with exponents, you must follow these 2 rules: (1) use the "^" symbol to mean "raised to the power of", and (2) omit all spaces between all elements in the expression, including units. The reason is that Blackboard grades your answer based on an exact match with the correct answer. If the correct answer is listed as 3.4x10^4cm^3 and you enter a variation such as 3.4  x  10^4  cm^3 or 3.4x  10 ^ 4 cm^ 3 or any other combination that includes spaces, Blackboard will grade your answer as WRONG!

 

The chemistry of life is based largely on the chemistry of water. When ionic or polar solutes are mixed with water, the water molecules (which are polar) attract them and may pull them apart into oppositely charged ions. This process is called dissociation. In fact, water molecules may even pull other water molecules apart to produce oppositely charged hydrogen ions (H+) and hydroxyl ions (OH-).
 

Use the interactive exercise below to learn more about the process of dissociation.

 

Because water dissociates into one H+ and one OH-, pure water always has the same concentration of H+ and OH-, which turns out to be 1 x 10 -7 M:

In pure water,

 [H+] = [OH-] = 1 x 10-7 M

NOTE: The brackets indicate “the concentration of” whatever is inside the brackets.


In addition, although solutes may alter the concentrations of H+ and OH-,  in all aqueous solutions the product of [H+] and [OH-] is always equal to 1 x 10-14 M:

In all aqueous solutions,

 [H+] x [OH-] = 1 x 10-14 M

Because of this relationship, for any aqueous solution it is possible to determine the concentration of OH- if one knows the concentration of H+ (or vice versa). Furthermore, it means is that as [H+] increases [OH-] must decrease and, of course, the opposite is true as well.

 

YOUR TURN
Each water molecule that dissociates produces          Hint Check your answer.
In pure water, [H+] =          Hint Check your answer.
In all aqueous solutions, [H+] x [OH-] =          Hint Check your answer.
If the [H+] of an aqueous solution equals 1 x 10-3 M, then [OH-] =          Hint Check your answer.
 

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