Question

Give a  chemical example, quantitivative analysis will tell you how much of something there is in your sample whereas qualitative analysis will tell you what is in your sample.

Answer 1

As an example, if you pour potassium thiocyanate (KSCN) in a solution and it goes blood red, it's sign that you have a iron III ions in that solution, according to the reaction

 

Fe+3+6SCN−→Fe(SCN)−36

 

In which the complex in blood red. This is qualitative because it tells you there is some amount of (free) iron III ions in that solution but doesn't tell you how much.

 

But if you pour an excess of NaOH in a solution with iron III ions, filtrate the caramel precipitate, calcinate it and weight it, you'll know how much iron III ions were in that solution according to the reaction

 

Fe+3+3OH−→Fe(OH)3(s)

 

2Fe(OH)3(s)→Fe2O3(s)+3H2O(g)

 

So by weighting the Fe2O3 you can know how much Fe+3 were there in the original solution and easily calculate concentration.

 

It's important to note though, that in a real life analysis you always do a qualitative one before a quantitative one. (Or at least someone else does and tell you the results).

 

This is because quantitative analysis need to only react with whatever your compound of interest is, so if you did that test I mentioned with the iron above in a solution that had Cr+3, which also precipitates, there'd be an error.

This is because quantitative analysis need to only react with whatever your compound of interest is, so if you did that test I mentioned with the iron above in a solution that had  Cr+3

 , which also precipitates, there'd be an error.