A student wishes to determine the concentration of sulfuric acid. They run a titration and discover that 62.5cm^3 is needed to neutralise 20cm^3 of sodium hydroxide of concentration 2moldm^-3. Work out the concentration of the acid.
To answer this question the first thing that needs to be done is to write a chemical equation, making sure it is balanced and includes state symbols:H2SO4 (aq) + 2NaOH (aq) --> Na2SO4 (aq)+ 2H2O (l)Next write out all of the numbers ensuring they have the correct units:Acid- V= 62.5 cm3 = 0.0625dm3 (to convert between cm3 and dm3 you need to divide by 1000)Base- V= 20 cm3 = 0.02dm3 and c= 2 moldm-3Next we need to work out the number of moles of sodium hydroxide:n= c x V= 2 x 0.02= 0.04 molNow we can work out how may moles of sulfuric acid we have:We can use the stoichiometry to get the ratio of acid to base (1:2) and use this to solve for the moles of acid:0.04/2=0.02 molRearrange the equation to get concentration and solve:c= n/V= 0.02/0.0625= 0.32 moldm-3We can also give this in gdm-3:0.32 moldm-3 = 31.36 gdm-3. To do this you need to convert moles into grams using the equation n=m/Mr. The Mr is 98 gmol-1 so by rearranging the equation to get mass=n x Mr we can find mass= 0.32x98= 31.36g dm-3
