does anyone know how to find the concentration of solutions when your given two volumes and two Molalities. like 200 mL of a .3Msolution of HNO3 and 500mL of a .4M solution of HNO3 are mixed together. what is the concentration of the resulting solution. im so lost:-(

First you work out how many moles of HNO₃ are in each solution (remember, molality is moles/kilogram, and water has a density of 1). Then you add up the volumes and the number of moles of HNO₃, and you can calculate the new molality.

 

So, in your example:

200mL of .3M HNO₃: [math]0.3 = \frac{x}{0.200}[/math]

Solving for x gives you the number of moles of HNO₃ in that sample. Do the same with the other sample, add them up, and calculate the new molality.

200mL of .3M HNO₃

 

M is usually used for molarity, not molality, so it can be a little bit confusing here.

 

Borek

That shouldn't matter too much in this example, but I was taught that M was molality...

A big M stands for molarity, a lower case-cursive m stands for molality. In the case of M(1)*V(1) = M(2)*V(2), we're dealing with molarity. Molality is moles of solute per kg of solvent, very different from molarity.

Crap.

 

Shouldn't make a difference in the final results of the example, though.

The example looks fine to me. Definitions and notation can get a bit tricky tho'.