Question

What is the pH of a solution formed by mixing 400 mL of 0.025 M HCl with 600 mL of 0.015 M NaOH?

Answer 1

mols HCl = M x L = ?

mols NaOH = M x L = ?
Subtract, the one in excess will determine the acidity/basicity.
If acid then pH = -log(HCl).
If basic then pOH = -log(NaOH), then convert to pH.