So wait @2:25 , why can we use the moles of glycerin without appending the ratio to water? Is it because it's assumed that it's there? So when you get 357.9 g of glycerin, is that to say that it's 357.9 g of glycerin per 1 kg of water? And then since you have a kg of water as the solvent you then add the 357.9 g of solute to get your 1357.9 g of solution>?
Yes. You need the total mass (from density) to get the actual volume - assuming you have that many moles dissolved in 1000 g of water. Then it's moles / L solution.