The pH of Acids

By Rebekah F. and Elizabeth H.

The pH Scale

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The pH scale measures the acidity or alkalinity of a solution and ranges from 0 to 14. The scale is based off the concentration of the positively charged Hydrogen (H+) ions within a substance. Acidic solutions fall on the scale between 0 to 6.9 and increase in intensity as they near 0. Basic solutions fall on the scale between 7.1 to 14 and increase in intensity as they near 14. A neutral solution is neither acidic nor basic and has a pH of exactly 7. Every number of the pH scale has a 10 times stronger concentration of H+ ions than the one before it, which means it is logarithmic.



Strong Acids


A strong acid is an acid that ionizes or dissociates completely in a liquid solution, and the concentration of H+ equals the concentration of strong acid added. The equation representing the dissociation of strong acids is:

HA(aq) → H+(aq) + A(aq) www.wikipedia.org
HA represents the strong acid before the reaction, H+ the hydrogen ions, and A- the anions.

Since the pH scale is logarithmic and the concentration of H+ ions equates to the the the activity of the strong acid, the pH is calculated using the formula:
pH = −log[H+]
www.wikipedia.org
As another method of measuring concentration, Normality can be used in place of the concentration of H+ ions in strong acids because the two are relatively equal.




Weak Acids



A weak acid is an acid that dissociates completely and does not release all its H+ ions in a solution. The equation representing the dissociation of weak is:

HA(aq) → H+(aq) + A(aq)www.wikipedia.org


The same equations is used to calculate the pH for strong acids is used for weak acids, but unlike a strong acid, Normality cannot be used in place of the concentration of H+ ions for weak acids. Instead, the concentration needs to be calculated using the Ionization Constant Formula:


www.wikipedia.org
www.wikipedia.org

HA represents the strong acid before the reaction, H+ the hydrogen ions, and A- the anions. The equation is set equal to the Ka, which stands for the Dissociation Constant.




Dissociation Constants


An acid's dissociation constant is a measure of the strength of that acid in a solution, and it is used to determine whether an acid is strong or weak. The smaller dissociation constant, the weaker the acid; the larger a dissociation constant, the stronger the acid.

A weak acid's dissociation constant is any number multiplied by 10⌃-3 or smaller. An example would be acetic acid, which has a dissociation constant of 1.8 X 10⌃-6 and makes it a weak acid.

A strong acid's dissociation constant is any number multiplied by anything greater than
10
⌃-3. An example would be hydroiodic acid, which has a dissociation constant of 3.2 X 10⌃9 and makes it a strong acid.




Practice Problems

1. .00751 N HI
2. .000631 N H(ClO)
3. .0000398 N H(CN)
4. .0200 M H(Cl)
5. .100 M Benzoic Acid
6. .025 M Hydrobromic Acid



Answers to Practice Problems


1. 2.1 pH
2. 5.3 pH
3. 6.9
pH

4. 1.7 pH
5. 2.6 pH
6. 1.6 pH




Demonstrations of Solutions



Practice Problem 1:



Practice Problem 2:






Common Dissociation Constants of Acids



Use these Constants to plug into the equations as shown in the demonstrations.


Acid


Equation


Ka Constant



percholric acid




HClO4




very large




hydroiodic acid




HI




3.2 X 10⌃9




hydrobromic acid




HBr




1.0 X 10⌃9




hydrochloric acid




HCl




1.3 X 10⌃6




sulfuric acid




H2SO4




1.0 X 10⌃3




nitric acid




HNO3




2.4 X 10⌃1




oxalic acid




H2C2O4




5.4 X 10⌃-2




sulfuruous acid




H2SO3




1.7 X 10⌃-2




hydrogen sulfate ion




HSO4




1.3 X 10⌃-2




phosphoric acid




H3PO4




7.1 X 10⌃-3




hydrogen telluride




H2Te




2.8 X 10⌃-3




hydrofluoric acid




HF




6.7 X 10⌃-4




nitruous acid




HNO2




5.1 X 10⌃-4




hydrogen selenide




H2Se




1.7 X 10⌃-4




benzioc acid




C7H6O2




6.6 X 10⌃-5




acetic acid




CH3COOH




1.8 X 10⌃-6




carbonic acid




H2CO3




4.4 X 10⌃-7




hydrogen sulfide




H2S




1.0 X 10⌃-7




dihydrogen phosphate ion




H2PO4




6.3 X 10⌃-8




hydrogen sulfite ion




HSO3




6.2 X 10⌃-8




hydrochlorous acid




HClO




2.9 X 10⌃-8




ammonium ion




NH4




5.7 X 10⌃-10




hydrogen carbonate ion




HCO3




4.7 X 10⌃-11




hydrogen telluride ion




HTe




1.0 X 10⌃-11




hydrogen peroxide




H2O2




2.4 X 10⌃-12




monohydrogen phosphate ion




HPO4




4.4 X 10⌃-13




hydrogen sulfide ion




HS




1.2 X 10⌃-15




ammonia




NH3




very small



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