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# Molarity vs. molality

Learn how molarity and molality differ! The molality of a solution is equal to the moles of solute divided by the mass of solvent in kilograms, while the molarity of a solution is equal to the moles of solute divided by the volume of solution in liters. For example, a 1 molal solution contains 1 mole of solute for every 1 kg of solvent, while a 1 molar solution contains 1 mole of solute for every 1 L of solution. Created by Sal Khan.

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• I thought when you dissolved something the volume didn't change. At minute that doesn't seem to be what he is saying.
• The volume does change when you dissolve something. You're putting molecules in -- the total volume must change, however slightly.

It doesn't mean that the total volume will increase exactly by the volume of solute you added.. That depends on the lattice structure of the solute (if solid), and the interactions with the solvent. A 1cm^3 crystal of NaCl dropped into 100cm^3 of water won't increase the volume to 101 because when it dissolves, the lattice structure breaks down and water will surround each molecule of NaCl (actually each atom of Na+ and Cl-) separately..

But at the end of the day, the volume does still increase because now you have all those atoms of Na and Cl floating around in there, and they do after all have some volume, even if it's tiny. It starts to add up when you add more solute..
• Anyone know where some good Molarity, mass percent, Dilutions exercises are to practice? (More beginner stuff)
• to the left of the video there is a thing that says practice molarity calculations
maybe you can try that?
• " 1 litre=1 kg"
Is this true only for water or we can use this relation for all other substances?
• 1l = 1kg = 1dm^3 for water only.
1l of oil is already less than 1kg
• I don't think this was very helpful. I'm trying to figure out how exactly to convert from l to kg or any other units that may be used in this problem. (Just a general comment not quite a question)
• To convert volume to mass, or vice versa, you first need to look up the density of the solvent you are interested in (at a specific temperature).. The density of water is 1 g/cm^3 (or 1 kg/L, since 1 cm^3 = 1 mL). The density of ethanol, for instance is 0.789 g/cm^3.. 1 L of ethanol = 0.789 kg (=1 L* 0.789 kg/L) . 1 kg ethanol = 1.267 L (=1 kg * (1/0.789 kg/L)).. For that last bit, just invert the density. If ethanol is 0.789 kg/L, it is also 1.267 *L/kg* (=1/0.789 kg/L).. By inverting the density, you go from kg/L to L/kg. That's all you need.

Now, when you add a dissolved solute into the mix, it complicates everything. Different solutes lead to different displaced volumes when dissolved in different substances. 1 gram of NaCl thrown into water might take up 1 microliter of space, but it might take up more or less space in some other solvent, maybe ethanol (it depends on the interactions between the molecules.. the geometry of the shells of solvent around the solute particles etc..). On the other hand, 1 gram of a big molecule like albumin will take up more volume than that.. It has to due with the density of the solute molecules individually, as well as their interactions with the solvent itself.. and I'm sure a host of other factors. I'd say that's something you pretty much have to measure every time..
• But isnt doing moles/.99 kg not the same as moles/1kg, therefore does not fit the definition of molality?
• Molality is a ratio of the number of moles of solute per kilogram of solvent. If you have .5 mol/.5 kg, the molality will be 1 molal, just as if you took 1 mol over 1 kg of solvent.
• Why 1 litre of a solution equals to 1 kg of the same solution?
• Because 1 liter is defined as the volume of water that makes up 1kg.
• Does this also apply to solid nonmetal solutes?
• If the solute dissolves in the solvent then it does because molarity is number of moles per litre of solution and a solution is formed only when solute dissolves in solvent
• Why is 1L of water = 1 KG?