A student titrates 25.00 cm3 of hydrochloric acid with 28.60 cm3 sodium hydroxide solution of concentration 0.200 moles per dm3. The equation for the reaction is: HCl + NaOH to NaCl + H2O. Calculate the concentration of the hydrochloric acid.

From the equation given in the question we can see that one mole of hydrochloric acid (HCl) reacts with one mole of sodium hydroxide (NaOH). 

The first thing to do is to work out the number of moles of sodium hydroxide that reacted by using the equation n = cv (number of moles = concentration x volume). 

To do this we need to convert the volume 28.60 cm³ into dm³ (which is the same as litres). There are 1000 cm³ in a dm³ so 

28.60 cm³ = 28.60 / 1000 dm³ = 0.0286 dm³

Now we can use n = cv = 0.200 moles per dm³ x 0.0286 dm³ = 0.00572 moles of sodium hydroxide

Since one mole of hydrochloric acid reacts with one mole of sodium hydroxide, 0.00572 moles of hydrochloric acid reacted.

We can now use the equation n=cv and rearrange it to c = n/v to find the concentration of hydrochloric acid. 

The volume of hydrochloric acid again needs converting to dm³:

25.00 cm³ = 25.00/1000 dm³ = 0.025 dm³

c = n/v = 0.00572 moles / 0.025 dm³ = 0.23 moles per dm³