Question

OVER 200 POINTS- THIS QUESTION IS POSTED TWICE ON MY PROFILE FOR 100 POINTS EACH AND IT IS POSTED A COUPLE TIMES FOR 30 POINTS. IF YOU KNOW THE ANSWER YOU CAN PUT IT ON ALL THE POSTS AND GET ALL THE POINTS I JUST REALLY NEED HELP. THANKS!

A student dissolves 0.250 mol NaC1 in 2.05 kg H20. What is the molality of the resulting solution? Show your work.

Answer 1

If all you are given is Molarity, then you also need to know the density of the solution to calculate molality, m (mol/kg solvent).

The easiest way to understand the process is to assume you have 1L of solution which is 1000 mL. Think of the units of molarity and molality. So here is an example:

What is the molality of a

3.00 M

solution of

NaCl

? The density of the solution is

1.12 g/mL

.

We know that in

1.00 L

of solution, there are

3.00

mol of

NaCl

, since that is the meaning of

3.00 M

.

If we figure out how many kg of water there are in

1.00 L

of this solution, we are almost done! Many students are confused between solution and solvent, so be careful.

First let's figure out the mass of the solution using dimensional analysis and density:

Answer 2

The molality ( m ) of a solution is the moles of solute divided by the kilograms of solvent. A solution that contains 1.0 mol of NaCl dissolved into 1.0 kg of water is a “one-molal” solution of sodium chloride. The symbol for molality is a lower-case m written in italics.

In order to calculate the molality of a solution divide the moles of solute by the volume of the solution expressed in liters.

I did what I think you meant if it's wrong text me I'll fix it.