Find the pH of 0.1M H3PO4 solution.
The pH of a solution is a measure of the molar concentration of hydrogen `(H^+)` , or hydronium `(H_3O^+)` ions of the solution. If a solution has a hydronium ion concentration of `[H^+]` , the pH of the solution is equal to `-1*log_10[H^+]` .
The pH of pure water at STP is equal to 7. Acids have a pH less 7.
When phosphoric acid `H_3PO_4` is diluted with water it dissociates as follows:
1) `H_3PO_4 -> H^+ + H_2PO_4^-`
2) `H_2PO_4^(-) -> H^+ + HPO_4^(2-)`
3) `HPO_4^(2-) -> H^+ + PO_4^(3-)`
The acid dissociation constants of (1), (2), and (3) are equal to 7.1*10^-3, 6.3*10^-8 and 4.5*10^-13 resp.
To determine the pH of a 0.1 M solution of phosphoric acid, the concentration of H^+ in the solution has to be determined. The first deprotonation step leads to the formation of 0.1 M of hydrogen ions.
The second and third deprotonation steps can be ignored for acid concentrations greater 0.001 M as the [H^+] due to these is negligible.
Now Ka1 = `([H^(+)]*[H_2PO_4^(-)])/[H_3PO_4]`
If we express `[H^+]` as X.
Ka1 = `(X*X)/(.1 - X)`
The value of Ka1 is `7.1*10^-3` .
Solving `7.1*10^-3 = (X*X)/(.1 - X)` for X gives 0.0233 as the positive solution of X.
The pH of the solution is `-1*log_10 0.0233` = 1.63
The pH of a 0.1 M solution of phosphoric acid is 1.63.
The pH scale is a scale from 0 to 14 that indicates the acidity of a substance. The number 7 is in the middle and represents neutrality, in terms of qualifying a substance as an acid or a base. Substances that are lower than 7 are classified as acids, meaning they have increasing numbers of hydrogen ions (H+) when dissolved with water. Substances that are higher than 7 are classified as bases, meaning they have less hydrogen ions and more hydroxide ions (OH-) when dissolved with water.
To find the pH of a .1 M sample of H3PO4, understand the .1 M is the same as 1 x 10-1. If we take the logarithm of 1 x 10-1, that would be expressed as - (-1), which would be 1. The pH of this solution would be 1, which would qualify it as a very strong acid, providing many hydrogen ions upon dissolution with water. Th pH scale is very useful in indicating the acidity or basicity of substances.
its a tripotic acid with three different Ka values they are Ka1, Ka2 and Ka3.
Ka1 = H3PO4 = 7.5 * 10^-3
Ka2 = H2PO4^- = 6.2 * 10^-8
Ka3 = HPO4^2- = 4.8 * 10^-13
Ka1>>Ka2 and Ka3.
which says that majority of Hydronium is produced during step one so we can ignore step 2 and 3.
Since H3PO4 is weak acid it will not dissociate completely and we have to write the ICE table
H3PO4 + H2O ↔ H3O^+ + H2PO4^-
I 0.10M 0 0
C -x +x +x
E 0.10-x x x
Ka1 = ([H3O^+][H2PO4^-])/[H3PO4]
7.5*10^-3 = ([x][x])/[0.10-x]
Find x usind qudratic equation...
x = 0.024M
[H3O^+] = x = 0.024M
PH = -log[H3O^+]
PH = -log[0.024]
Ph = 1.619