n = m/Mr (n= number of moles of a compound; m= mass of a compound; Mr = formula mass of a compound)
Rearranged: m = nxMr
Using the balanced equation for the reaction; Ca(OH)2 + 2HCl ---> CaCl2 + 2H2O we can see that there are 2 moles of OH- ions for every 2 moles of HCl and therefore we need to react the same number of moles of HCl as moles of OH- ions:
n, OH- ions = 2.6x10^24 mol
therefore:
n, HCl = 2.6x10^24 mol
Mr, HCl = 36.5 (use periodic table and the molecular masses of each element in the compound to find)
To calculate the mass of HCl needed, we multiply the number of moles of HCl and the Mr of HCl:
(2.6x10^24) x 36.5 = 9.49x10^25 g of HCl is required
