# Concentration of Solutions

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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:-(

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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.

200mL of .3M HNO3: $0.3 = \frac{x}{0.200}$

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.

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200mL of .3M HNO3

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

Borek

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

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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.

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Crap.

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

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

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