unit 3 questions

[bec]

anyone know how this would work?

"Anhydrous sodium carbonate is used as a primary standard in determining the concentration of hydrochloric acid by volumetric analysis."

i understand titrations when there's a base like NaOH or KOH (one with a hydroxide group) but how would something like sodium carbonate be used to measure the concentration of HCl?

[cara.mel]

It's a base as well, reacts according to Na2CO3 + 2HCl -> 2NaCl + H2O + CO2
(carbonates are part of your acid + whatever reaction rules you learnt last year)

The reason it's being used over NaOH etc is because it meets all the checkpoints of being a good primary standard, ie it doesn't react with atmosphere over time, has a known formula etc (there's about 5 or 6 reasons but I've forgotten them all)
So you can get a accurate concentration of HCl because you can accurately determine the concentration of your Na2CO3 when you measure it out and make it etc

Hope that makes sense, if it doesn't ask

[bec]

thanks for that! and yes it does make sense (or at least i think it does..)
so once the equivelance point is reached, we know that there's an exactly equal amount of Na₂CO₃ and HCl? is that right?

[cara.mel]

From the equation, at the equivalence point there will be twice as much HCl as Na₂CO₃. you always need to write out an equation and know how to balance it.

So yeah, main reason is that you can be dead sure what the concentration of Na₂CO₃ is (because it is a primary standard) compared to other compounds

[Collin Li]

Yeah, the equivalence point is a stoichometric balance.

[bec]

oops yeah i knew that, don't know why i wrote what i did!
thanks heaps though, i get it now!