Phosphoric acid has a chemical formula of H3PO4.
That means, each mole of phosphoric acid contains 3 moles of hydrogen atoms. Also, each mole of a substance contains an Avogadro's number of atoms or 6.023 x10^23 atoms.
Here, we have 4.77 x 10^30 atoms of hydrogen.
We can calculate the moles of hydrogen using the knowledge of Avogadro's number.
Hence, moles of hydrogen = (4.77 x 10^30)/(6.023 x 10^23) = 7.92 x 10^6 moles.
Using the chemical formula of phosphoric acid,
3 moles of hydrogen atoms are present in 1 mole of phosphoric acid.
7.92 x 10^6 moles are present in 2.64 x 10^6 moles of phosphoric acid (= 7.92 x 10^6 / 3)
The molar mass of phosphoric acid is 98 grams.
Therefore, the amount of phosphoric acid that was dissolved = 7.92 x 10^6 x 98 gm
= 2.59 x 10^8 gm.
Hope this helps.