 Now, we are already familiar with the various ways in which concentration of a solution can be expressed, like molarity, molality, mole fraction, mass percentage, volume percentage and so on. So, in this video, we are going to solve a few questions based on these concentration terms. Okay? So, let's look at the first question which states that, a chemical solution contains 23.5% biomass phenol in water. If the density of the solution is 0.995 gram per ml, what is the molarity of the solution? So what do we need to find here? We need to find the molarity of the solution. And what is the formula for molarity? We know that molarity is nothing but number of moles of solute divided by volume of solution in litre. Okay? But do we have any information directly on the number of moles of solute or the total volume of solution here? Well, not directly, but they are definitely there in the question. You see, when we say 23.5% by mass phenol, what do we mean by that? It means we have 23.5 gram of phenol in 100 gram of solution. Now, how do we arrive at the number of moles of solute from this mass? We know that number of moles of phenol is nothing but given mass of phenol divided by the molar mass of phenol. Right? Now, phenol is nothing but C6H5OH. Now from this particular formula, you can easily arrive at the molar mass which is 94 gram per mol. So now we have 23.5 grams divided by 94 gram per mol which is the molar mass of phenol and this gives us 0.25 mol. So this can be substituted in our numerator. What about our denominator? How do we arrive at the volume of the solution from mass of the solution? Simple. We have density. Now density is equal to mass by volume so volume will be mass by density. Now mass of the solution is 100 gram and density is given as 0.995 gram per mol. On solving this we get 100.5 ml which is nothing but 100.5 into 10 power minus 3 litre because we are looking at volume of the solution in litre. So that's it. We have the number of moles of solute here and we also got the volume of the solution. All we need to do is simply substitute these two values in the molarity equation. And when we do that we get 0.25 mol divided by 100.5 into 10 power minus 3 litre which is equal to 2.49 molar. Let's now look at a slightly trickier question. So here the question says what mass of solid NaOH 90 percent NaOH per mass is required to prepare 1 litre of 20 percent NaOH solution by mass and the density of the 20 percent solution is given here. Now this might sound very confusing but don't worry let's break it down into smaller chunks. Now in order to find out the mass of solid NaOH required we first need to know what is the mass of this 20 percent NaOH solution. Now mass of solution can be obtained from the details like volume and density here because mass is nothing but density into volume which is 1.18 gram per ml and what is the volume of the solution here? It is 1 litre. So if we change into ml that would be 1000 ml. So ml-ml gets cancelled and that means the mass of our entire solution is 1180 grams. Now if this is the mass of the solution then how much is the amount of solute present in it or what is the mass of NaOH in this amount of solution? Well that's pretty straightforward because what do we have here? It is said that it is a 20 percent NaOH solution which means 20 grams of solute is present in 100 grams of solution. Now what we have here the total mass of solution is 1180 grams. So 1180 grams of solution contains how much solute? Well on cross multiplying we get the final answer as 236 grams of NaOH. So that's it. Have we arrived at the final answer? But wait a minute. We don't have 100 percent purity. The solid NaOH that is given to us is only 90 percent pure. So we have to take into account the purity difference here. That means the actual mass of NaOH required would be greater than whatever is obtained here. Right? Because it's only 90 percent pure. So actual mass of NaOH would be 236 grams divided by taking into account the 90 percent purity 0.90 that is 262.2 grams. So this is the actual mass of NaOH required to prepare 1 liter of 20 percent NaOH solution by mass.