How many moles of H3O+ or OH- must you add to a liter of strong base solution to adjust its PH from 9.870 to 7.980. Assume a negligible change in volume.

Answer 1

Convert pH = 7.980 to (H3O^+) = ? mols/L.
Convert pH = 9.870 to (H3O^+) = ? mols/L.
Subtract to see the difference in mols.
The pH is now 9.87 and you want to move it to 7.98, therefore, you must add acid and you want to add the mols difference.
Sub