A student reacts 50.0cm^3 of 2.00mol dm^-3 HCl with 25.0cm^3 NaOH. What is the concentration of NaOH?

n(HCl) = 0.05 dm^3 x 2 mol dm^-3 = 0.1 mol 

Ratio HCl : NaOH = 1 : 1 = 0.1 mol : 0.1 mol 

c(NaOH) = 0.1 mol / 0.025 dm^3 = 0.4 mol dm^-3