How do you perform titration problems to find concentration?

1 Answer
Aug 20, 2017

The basic equation is #"Concentration"="Moles of solute"/"Volume of solution"#


#"Concentration"="Moles of Titrant"/"Volume of titrant"#, i.e.

#C=n/V#........and remember this if you remember anything here. It is consistent dimensionally, i.e. #mol*L^-1=("moles")/(L)#

And thus a measured volume of titrant of KNOWN concentration is added to an unknown quantity of titrand, with which it reacts in known stoichiometric proportion. Some sort of indicator is added so that the stoichiometric endpoint is precisely revealed.

All of this is unuseful UNLESS you have a stoichiometrically balanced equation to inform you of what is going on in solution.

#NaOH(aq) + HCl(aq) stackrel("indicator")rarrNaCl(aq) + H_2O(l)#

Here, in an acid base titration, sodium hydroxide and hydrochloric acid are added, and these react in a 1:1 ratio. The moles of #NaOH# are unknown; the VOLUME and the CONCENTRATION of the hydrochloric acid titrant are known, and the 1:1 stoichiometry allows direct measurement of the number of moles of #NaOH#.

On the other hand redox titrations can be performed in which an analyte is oxidized by an oxidant of known concentration:

#2underbrace(MnO_4^(-))_"deep purple"+6H^++5HO(O=)C-C(=O)OH(aq)rarr 2Mn^(2+) + 10CO_2(g)uarr+ 8H_2O(l)#

Here oxalic acid to OXIDIZED to give carbon dioxide in a precise stoichiometric ratio by potassium permanganate. That #KMnO_4(aq)# is intensely purple, and #Mn^(2+)(aq)# is almost colourless, allows determination of the concentration oxalic acid provided that the the concentration of oxalic acid is known.

A third type of titration occurs when the titrant forms an insoluble precipitate with the titrand. The Mohr titration is used to determine concentration of halilde ions.....

#Ag^(+) + X^(-)stackrel(Na_2CrO_4)rarrAgX(s)darr#

The endpoint is reached when insoluble, and BRICK-RED #Ag_2CrO_4# precipitates from solution.

SO, I don't really know what you want, but remember the basic equation defining concentration. You also need a stoichiometrically balanced equation that represents the reaction.