Find the pH of 0.1M H3PO4 solution.

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sanjeetmanna's profile pic

sanjeetmanna | College Teacher | (Level 3) Assistant Educator

Posted on

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

bandmanjoe's profile pic

bandmanjoe | Middle School Teacher | (Level 2) Senior Educator

Posted on

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.

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