Qualitative analysis

Question

39.8g of a mixture of KCl and KClO3, were heated on a constant mass.  If the residue weighed 28.9g, what was the percentage mass of KCl in the mixture.  

 

Note.  KCl is not decomposed on heating 

Answer 1

39.8g of a mixture of KCl and KClO₃, were heated on a constant mass.  If the residue weighed 28.9g, what was the percentage mass of KCl in the mixture.

 

2KClO₃ --> 2KCl + 3O₂

 

The mass difference is oxygen released

39.8g - 28.9g = 10.9g O₂

 

10.9g O2 * 1 mole / 32 g = 0.3406 mol O₂

Using stoichiometry

0.3406 mol O₂ * 2KClO₃ / 3O₂ = 0.22708 mol KClO₃

multiply the molar mass of KClO₃ by the mole KClO₃

0.22708 mol KClO₃ * (39.1g + 35.45g + 3*16g )= 0.22708 mol * 122.55 g/mol KClO₃ = 27.84 g KClO₃ was in the original mixture.

So..

 

39.8g mixture - 27.83g KClO₃ = 11.97g KCl

 

Percentage mass:

( 11.97g KCl / 39.8g mixture ) * 100 = 30.1% KCl