What volume of 0.0500 M Ba(OH)2 will react completely with 27.50 mL of 0.550 M HCl?

Answer 1

Ba(OH)2 + 2HCl ==> BaCl2 + 2H2O

mols HCl = M x L
Mols Ba(OH)2 = mols HCl x (1 mol Ba(OH)2)/2 mols HCl).
M Ba(OH)2 = mol/L. You have M and you have mols, calculate L. Check my work. It's getting late.