posted by jacob on .
i don't really understand molality. I know that it is a big concept but could someone explain it a little.
the question is how would you prepare a molality solution.
Molarity is about as simple as it gets. The definition is all you need.
The definition of molarity is
Molarity = M = # moles/Liter of solution.
So, to prepare a 1 molar solution, of something, say NaCl, you dissolve 1 mole of NaCl in water, shake it around until all of it dissolved, then make the final volume of the solution to 1 L. Simple, eh what? A 2 molar solution you dissolve 2 moles of NaCl in ......, a 3 M by dissolving 3 moles NaCl.... etc.
What is a mole of NaCl? moles = g/molar mass. So 1 mol of NaCl is about 23 for Na and about 35.5 for Cl or about 58.5 grams for a mole NaCl. (You can look up Na and Cl on the periodic table and get the exact mass if you want it). Since M = mols/L, that can be rearranged to make M x L = moles and L = mols/M.
Please let me know if this doesn't do the job BUT be sure and tell me exactly what is still unclear about it.
thanks i didn't really get molarity either but my question is for molality.
I didn't answer your original question because it makes no sense. There is no such thing as preparing a molarity solution. You can prepare a MOLAR solution or a TWO MOLAR solution but you can't prepare a molarity solution. You CAN add an maount of material to a certain amount of water and ask for the molarity of that solution and it's th same concept. Its mols/L = Molarity. So if you are given 29.25 g NaCl in a final volume of 500 mL (0.50 L), then the solution is M = moles/L. We have 29.25 grams NaCl which is 20.26/58.5 = 0.500 g in 0.500 L = 0.5/0.5 = 1 M.