 In this video, let us review the various terms we use when talking about concentration of solutions in chemistry. So the first one is mass percent. Let's say I'm adding 4 grams of NaCl into 16 grams of water and this gives me a solution. Then this 4 gram of NaCl, this NaCl is what we call the solute which is yellow here and this water in which we are dissolving NaCl is what we call the solvent. So the water that is the dissolving medium here is what we call the solvent and the substance that is dissolved that is sodium chloride here is what we call the solute. I keep forgetting these words that's why I have color coded them here for my convenience. So this NaCl here is the solute. So if I calculate the mass percent here, so I have 4 grams of NaCl, so that is the mass of solute divided by mass of solution. Will it be 16 grams? No, it won't be 16 grams. 16 grams is the mass of the solvent water. We added it to 16 grams of water and after adding it, the solution will then be 20 grams. So the mass of the solution here is 20 grams and not 16 grams. So be careful while performing this calculation and we multiply it with 100% and this will give us 5 and 20. So this will give us 20%. So the mass percent of this solution is 20%. So the mass percent of the solute in this solution is 20%. That is there is 20% solute by mass in this solution. Alright, let's look at the second term now. The second term is mole fraction. So let us say mole fraction of A. Mole fraction of A which can be solute or it can be the solvent is the number of moles of A divided by the number of moles of the solution. Let's do this calculation for this case here. So here we have NaCl as our solute and let's calculate the mole fraction of the solute NaCl. So first we need to find the number of moles of A. So number of moles is given mass divided by the molar mass. So given mass here is 4 grams and the molar mass of sodium chloride will be 23 for sodium and 35.5 for chlorine. This ratio will give us the molar mass. We represent it with Na. Similarly we can find the number of moles in the solution. So for the solution we need to add the number of moles in the solute with the number of moles in the solvent. So let's also calculate the number of moles in the solvent. So let's call the solvent B. Let's call water B here. So number of moles in the solvent will again be given mass which is 16 grams divided by the molar mass of water. Molar mass of water is 2 plus 16. 2 into 1 for hydrogen so 2 plus 16 for oxygen grams. So this is 16 by 18. These are the number of moles of water. So now that we have Na that is number of moles of the solute and NB which is the number of moles of the solvent. We can now find the mole fraction. The mole fraction will be Na the number of moles of A divided by the number of moles in the solution. So in the solution we have both Na and NB. So this will be the mole fraction of A. Okay. All right. Just as before don't forget in the denominator we have solution and not just the solvent. Okay. Let's go to the third one now. The third one is molarity which is represented by capital M. So the numerator here stays the same. It is the number of moles of the solute which here again is Na 4 divided by 58.5 in this case divided by now the volume of the solution in liters. We don't know the volume of the solution here. Let's say that the volume of the solution is 200 ml for example. In that case the molarity here would be number of moles of solute which is our Na which we calculated previously divided by 200 ml. Is it? No. We have to remember that the volume of the solution should be in liters. So this will write as 0.2L and this number will give us the molarity of the solution. Remember to write the volume of the solution and that too in liters. Okay. Finally we have molality which sounds very similar but it's represented by a small m and the definitions are different. Here too the numerator has number of moles of solute. So again Na in the numerator the number of moles of the solute but now we are dividing with the mass not the volume the mass of the solvent. This is the only one that talks about the mass of the solvent and not solution. So we're talking about solvent in the denominator here not solution. So this is the hard thing about molality in this group that it is the only one which has solvent in the denominator. So remember to use only the mass of the solvent here which in this case is 16 grams. So we will write 16 divided by 1000 because we have to make sure it's in kilograms. So take care of the units 16 divided by 1000. So if we divide this then what we'll get is what we call the molality. So these are the four concentration terms that are used in chemistry. Molality is the most popular one but we need to know all of these.