Phosphoric acid has a chemical formula of `H_3PO_4` .
Assuming it dissociates completely, it will release 3 moles of hydrogen cations for every mole of phosphoric acid dissolved, according to the following chemical reaction:
`H_3PO_4 -> 3 H^+ + PO_4^3-`
Here, 3.4 moles of phosphoric acid were dissolved.Thus, using the stoichiometry and the above mentioned dissociation equation for phosphoric acid, the number of moles of hydrogen cations released are 3 x 3.4 = 10.2 moles of `H^+` .
Each mole of any substance contains an Avogadro's number of atoms, that is, 6.023 x 10^23 atoms.
Here, 10.2 moles of hydrogen cations are in the solution.
or, 10.2 x 6.023 x 10^23 hydrogen cations are in the solution.
Solving this, we get the number of hydrogen cations in the solution as 6.14 x 10^24.
Hope this helps.