Titrate the acid of known concentration against the alkali of unknown concentration to find the volume of acid needed to neutralise a known volume of the alkali. Remember to use an appropriate indicator and perform a test run so you can titrate in small steps close to the end point. Only accept your result when you have three concordant results. (Results that are close to one another, within plus or minus 0.5 cm3.)When the volume has been found, write out the neutralisation reaction between the alkali and acid. Use this to work out the ratio of acid to alkali and then calculate the concentration of the alkali. Remember to always include units in every step so you don't get lost!