The ammonium ion is NH4+ which should have a molecular weight of 18g/mol. Then, the amount of ammonium ion in mol should be 7.65g/ (18g/mol)= 0.425 mol.
The formula for ammonium sulfate would be (NH4)2SO4. For every ammonium sulfate, you need 2 ammonium ion. Then the amount of ammonium sulfate in mol should be: 0.425 mol * (1/2)= 0.2125mol
The volume of the solution would be: 0.2125mol/ (0.812 mol/liter)= 0.26169l= 261.70 ml
