A 25.0 mL solution of HCl has been spilt. It took 21.8 mL of saturated 2.5 M NaOH to neutralise the acid. What was the molar concentration of the acid?

HCl + NaOH ==> NaCl + H2O

mols NaOH = M x L = ?
mols HCl = mols NaOH (you can see 1 mol HCl = 1 mol NaOH)
Then M HCl = mols HCl/L HCl