Acid base titration problem. Need Help

[ddluu]

How many moles of H3O+ must be added to one liter of a solution to adjust the pH from 7.17 to 5.85?

Correct answer 1.34E-6 (1.34 * 10-6)

The correct answer is given, however I would like to know how to do this problem.

 

[Astronuc]

See definition of pH - http://www.chembuddy.com/?left=pH-calculation&right=pH-definition

The pH is related to the molar concentration.

pH = 7.17 yields a molar concentration of 10^-7.17 = 6.76 x 10^-8 M, or moles/liter

Now try to find the number of moles in one liter for pH = 5.85.

Take the difference which gives the number of moles of H⁺ or H₃O⁺ one needs to add to one liter.