According to the law of definite proportions any two samples of KCl have?

1 Answer
Jun 30, 2015

The exact same proportion of potassium and chlorine by mass.


According to the Law of Definite Proportions, a chemical compound will always have exactly the same proportion of elements by mass.

This means that the elements that make up a compound will always have the same percent composition by mass, regardless of the actual mass of the sample.

In your case, potassium chloride has a molar mass of 74.551 g/mol. The two elements that form potassium chloride are potassium, which has a molar mass of 39.0983 g/mol, and chlorine, which has a molar mass of 35.4527 g/mol.

This tells you that every mole of potassium chloride weighs 74.551 g, out of which 39.0983 g is potassium and 35.4527 g is chlorine.

Therefore, chlorine and potassium will always be in a ratio by mass of

#(35.4527cancel("g"))/(39.0983cancel("g")) = 0.9068/1#

Simply put, you get an almost equal contribution to the mass of the compound from both elements.

Another way of looking at this is by calculating each element's percent composition.

#"For Cl": (35.4527cancel("g/mol"))/(74.551cancel("g/mol")) * 100 = 47.55%#

#"For K": (39.0983cancel("g/mol"))/(74.551cancel("g/mol")) * 100 = 52.45%#

This means that every sample of potassium chloride, regardless of its mass, will contain 47.55% chlorine and 52.45% potassium.

So, as a conclusion, any two samples of potassium chloride will contain potassium and chlorine in a 0.9068 : 1, or 52.55% to 47.55%, mass ratio.