Question

200. mL of a 0.50 M NaOH solution requires 100.0 mL of a HCl solution to

neutralize it. What is the concentration of the HCl solution?

Answer 1

So the first thing we must do is write a balanced equation for the reaction and we know the equation is balnced when all the species on the RHS is equal to the species on the LHS
NaOH + HCl → NaCl + H₂O

So now it's time to identify what you know from the question (volume & conc. of NaOH) and use that info to find the unknown (conc. of HCl)

If 1L (1000ml) of NaOH contains 0.50 moles then

let 200ml of NaOH contain x moles

⇒ x =
`(200 ml * 0.5 mol)/(1000 mol)`

= 0.1 mol

∴ moles of NaOH that reacted with HCl is 0.1 mol

Now mole ratio of NaOH : HCl based on the balance equation is 1 : 1
1NaOH + 1HCl → NaCl + H₂O

∴ if moles of NaOh = 0.1 mol
then moles of HCl = 0.1 mol

<<Alright Now to find the molarity of HCl>>

If 0.1 moles of HCl (which was what took place in the reaction) is in 100 ml
then let x moles would be in 1000ml (1L)

∴ x =
`(0.1 mol * 1000 ml)/(100 ml)`
= 1 mol

Thus the concentration of the HCl solution is 1 mol/L

PS. what is in bold is what you really need, the rest is just to help in understanding.