Calculate the molaLity of 36% by mass HCl.

Question

This is a concentration of solutions problem. I don't know what is supposed to be the kg in the malamute equation. I converted 100 g to kg  because I was assuming it was 100 g ?

Answer 1

This is one of the critical steps when converting solution concentration units - keeping everything in ratio form can get confusing (in my opinion) so it is easier to assume some sample size.  In this case, since you have 36% by mass, you know that you have 36 g HCl in 100 g solution.  

 

The most common mistake students make in these types of conversions is to throw around conversions for g and kg and mL but not LABEL those as "g of something".  

 

You are trying to find molality.  What is molality?  The units are "moles per kg", but you have to know WHAT it is that you are talking about.  Moles of WHAT per kg of WHAT?  Molality is moles of SOLUTE per kg of SOLVENT. Missing this important distinction will have you running in circles in conversion problems... 

 

You said you converted 100 g to kg, but if you do that, then you are saying you have 100 g of solution which is 0.100 kg of solution.  Solution is the solute PLUS the solvent - it is the mixture!  This means you absolutely do NOT have 0.100 kg of solvent!  

 

So how do we approach this?  Assuming 100 g is fine, but 100 g of solution means that you have 36 g of HCl (solute) and then how much solvent do you have?  It is the rest of the 100 g!  So 100 g - 36 g HCl = 64 g H₂O (solvent).  

 

Do you think you can work it from there?