When calculating the vapour pressure, wouldn't you also have to use Henry's Law to calculate the vapour pressure exerted by the solute as well, so that you can add the vapour pressure exerted by the solvent to that exerted by the solute to get the total?
When calculating the vapor pressure we are using a non-volatile solute in this example so that we don't have to add that in. Otherwise we would have to calculate the vapor pressure of the solute as well.
Not with colligative properties. However, if you look at a phase diagram of water, the freezing point of water can be changed slightly by increasing or lowering the applied pressure. But not by anywhere near 10 degrees Celcius. If you want to *lower* the freezing point, a few molal salt solution would do the trick.
you would solve for m, in Delta T = m * kb. Rearrange for the m, m = T / kb = 10C / 0.51 = 19.6 m of particles or with glycerin, 19.6 moles per L of water (a lot).
All of the calculations in the video assumed that the solute did not dissociate. But what if we have like MgCl2 in water? The adjustment is pretty simple - colligative properties depend on the concentraion of all the particles togther. So if you had 0.10 M MgCl2, then the Molarity for the problems would be 0.30 M since MgCl2 ---> Mg 2+ and 2 Cl-, three particles. So for example you'd end up with three times the freezing point depression, boiling point elevation and osmotic pressure and 1/3 or so the vapor pressure.
It is true M (big M) stands for molarity and m stands for molality. I went back and looked and with my handwritting, that was a "little m". When I write a big M it has hard corners at the top peaks and my little m has softer shoulders. When doing BP or FP, you must use molality (little m) which is mole / kg solvent . Molarity (big M) moles / L solution is used for osmotic pressure , and "X" (mole fraction) is used for vapor pressure.
If you look at 14.16 min you will see that I did indeed take ratio of moles *solvent* to moles of everything else. When calculating mole fraction, we are using the volatile part, in this case the water which is moles *solvent*. The solute in this case has no vapor pressure to contribute.
