 Let us discuss one of the topics in which concept based MCQs are asked and they are quite difficult to answer because it's kind of maths in one day and I see students fumble in these concepts. So, we will discuss about moles, millequivalence, osmosis and osmotic pressure. What is moles? Moles is basically gram molecular weight of substance. That means what is the molecular weight of the substance that is equal to 1 mole. So, suppose if we consider NaCl, right? Molecular weight of sodium is 23 and molecular weight of chloride is 35.5. So, if we consider only maybe sodium then this is 1 mole. If for chloride it is 35.5. But one mole of NaCl will be how much? It will be 23 plus 35.5 that is the we are adding the molecular weights of sodium and chloride. So, it will come to 58.5. So, this is the gram molecular weight of NaCl and it is equivalent to 1 mole of NaCl. And remember that 1 mole of any substance contains 6 into 10 to the power 23 molecules of that substance that is the Avogadro's number. Coming to next concept that is the equivalence. What is equivalence? Well, equivalent is used only for ionized substance, okay? So, suppose the weight is given for sodium it is 23 and for calcium it is 40. We said it is mole, right? Now, to calculate the equivalent weight, weight, not milli equivalence we are not talking that, equivalent weight of each of these. We have to divide the mole, 1 mole of ionized substance. We have to divide it with valency of that substance. So, if we calculate the equivalent weight for calcium, how much it will be? 40 divided by 2 is equal to 20 and for sodium it will be same 23 because the valency is 1 only, okay? So, that is equivalent weight. But remember calculation of milli equivalence that how many milli equivalents are there? It is different. We have to multiply it with the moles. See, because equivalent weight is half, isn't it? So, if we see how much concentration in milli equivalence per liter is there? Because the molecular weight is half, the concentration will be doubled, isn't it? Fine. Let's go to next concept that is the normality. What is normality? Normality is basically number of gram molecular weight in 1 liter. So, if we have to prepare a solution of 1 and Nacl, that means we have to add 58.5 grams in 1 liter. So, that will become 1 and Nacl, right? And let us see another example of that. How much HCl to be added to create 2N solution of HCl, right? So, 2N solution, that means we need 2 gram molecular weight in 1 liter or 2 moles of the substance in 1 liter. So, 1 mole is how much? 1 mole will be, these are the molecular weights given, it will be 35.5 that is the HCl plus 1 for hydrogen. So, it will give you 36.5 grams, right? So, we have to double it and how much it will be? So, 73 grams of the substance when added in 1 liter of the solution will give you 2N solution of HCl. Fine. Let's move on to next concept that is the osmoles. What is osmoles? Osmoles is number of moles, right? Into number of freely moving particles released after dissociation. So, 1 mole of Nacl, 1 mole of Nacl. Now, if Nacl dissociates, how many particles will be there? It will be sodium and chloride, okay? So, 2 particles, 1 mole of Nacl is releasing 2 particles. So, that will be 2 osmoles, 1 mole of Nacl will be equal to 2 osmoles or 1 millimole of Nacl will be equal to 2 milliosmoles. We should know about this moles and milliosmoles because when we are talking in physiological conditions, generally we will see that the concentration of various substances is in millimoles and we are talking about osmolarity in terms of milliosmoles, okay? So, read the question carefully when you are trying to understand it, moles, millimoles, what it is written. So, let us see another question. Calculate number of osmoles for 1 mole of Nacl and Cacl2. So, I mean these are two different things, right? So, if we calculate for Nacl, it will be 2 osmoles and for Cacl2, Cacl2 will dissociate into 3 particles. So, it will cause 3 osmoles. So, it is very simple, the calculation of the osmoles. Then what is osmolarity? Osmolarity is number of osmoles per litter of solution. Then there is another term that is osmolarity. Osmolarity is number of osmoles per kg of solvent. Two terms are there, right? Osmolarity. So, osmolarity is litter. I remember it is like how verbally it is being pronounced. Larity, litter, both have RR in their pronunciation. But litter of solution, that means it is taking into account the solvents also and solute also. Solution has solvent and solute. So, it is taking into account both and osmolarity is osmoles per kg of only solvent, okay? And this is much better to use osmolarity because osmolarity also depends on the temperature. When temperature changes, actually the osmolarity changes because it is taking into consideration solute also. So, the movement of the particles differs, right? So, osmolarity is the one which is used. So, why it is important to know the concept of osmolarity? Because more the osmolarity, more the solution will draw in water, right? So, this concept we have discussed in osmosis also that how osmolarity is important for passive diffusion of water. So, suppose this is A and this is B and A side is having certain solutes which are not able to cross the membrane and the membrane is only permeable to water. So, that is very important that membrane should be permeable only to water. Solute should not be able to cross the membrane and reach to the other side. So, water will come from B to A and this side A side will rise in level. So, this solute particle which is not able to cross the membrane, these are known as effective osmosis. On the other hand, suppose there was a solute which is able to cross the membrane. So, what will happen that this it is going to cross and it will equally distribute on both the sides, okay? In that case, there is no net movement of water because it is solute which will equally distribute on both the sides, right? So, these solute is known as ineffective osmosis. Exaptors are glucose, urea in body, right? Now, coming to how to calculate how much is the osmolarity? I am talking that, okay, there is some solute particles there. But if we have to calculate how much is the osmolarity, how to do that? Well, there are two mechanisms to do that, right? One is by freezing point depression. One is by freezing point depression and this is the one which is most accurate, okay? Freezing point depression because one osmol of the solute actually causes the depression of the solution by 1.86 degree Celsius. So, remember this one osmol causes the depression of freezing point of the solution by 1.86 degree Celsius, yes. So, by noting down that how much a depression in the freezing point has occurred, we can actually determine the osmoles. Then there is another method, the formula is there and the formula states that twice of sodium and potassium in milli equivalence per liter that is the concentration. Then 1 by 18 of glucose concentration in milligram per deciliter plus 1 by 2.8 or 0.36 multiplied by the concentration of blood urea nitrogen. So, three things have been taken into account. There is ions, there is glucose and there is blood urea nitrogen when we are determining osmolality using this formula. And remember that here when we are using this formula, the units which I have said is milli equivalence per liter, glucose it is milligram per deciliter and blood urea nitrogen it is milligram per deciliter. But suppose if the units given are in millimoles per liter, then you just have to add all the values. So, if all the units are given in millimoles per liter, then you just add all the values and you will get the osmolality in milliosmoles per liter. And how it is that millimoles get converted to milliosmoles, simple. You see here as I told you here it is already put twice because this formula why we are putting twice because we are taking into consideration the anions also, is not it? One sodium will have one associated anion also like chloride. So, we are taking the concentration of cations and doubling it so that anions are also taken into consideration. So, automatically that millimoles is getting doubled is not it? And for glucose in blood urea nitrogen, valency is what? Valency is one only right? So, the osmoles or the milliosmoles will be equal to the millimoles. So, that is why you just add the values of these. But one thing you remember here that the osmolality which is determined by these two mechanisms that is by the formula and by the freezing point depression, it differs. The osmolality determined by freezing point depression is more right? It is actually greater than 10 milliosmoles per liter approximately 10 milliosmoles per liter differences there. But if the difference is much more that means osmolality determined by freezing point depression say suppose is 320 and that determined by osmolality formula is suppose a 290. So, the difference is much more that means there is some external substance apart from this sodium potassium glucose and blood urea nitrogen which is causing difference in osmolality. Okay and this difference between the osmolality determined by freezing point determination and freezing point depression and osmolality formula it is known as osmolal gap. So, this helps us to determine that if there is any presence of any foreign substance. Coming to next concept osmotic pressure. What is osmotic pressure? Osmotic pressure is calculated by means of a formula nRT by V where n is the number of the osmoles right V is the volume of the solution. So, this is basically the concentration of the osmoles and R is the gas constant and T is the absolute temperature. Okay and this will give you the osmotic pressure. Now no need to remember these values of gas constant temperature and actual calculation of everything but if we simplify this right if we take this concentration value as 1 osmoles per litter and because these two are constants in a physiological condition. So, what happens that the osmotic pressure which is determined comes to 19300 millimeter mercury. So, 1 osmol per litter exerts an osmotic pressure of 19300 millimeter mercury or we can say 1 milli osmoles per litter we are always talking in milli osmoles because in body the osmolarity is in milli osmoles not in osmoles right. 1 milli osmoles per litter exerts a pressure of 19.3 millimeter mercury. So, if we know what is the concentration of the solutes right the osmoles if we know we can determine the osmotic pressure how much osmotic pressure needs to be exerted to stop the osmosis. So, simply we will put in this formula milli osmoles per litter into 19.3 and what is this reflection coefficient we have to multiply this by reflection coefficient because if you see that before I have talked about effective osmoles and ineffective osmoles that means if there is a osmol which can cross both sides isn't it. So, initially it may be that it may exert some osmotic pressure, but it will cross both sides and equilibrate in that case the osmotic pressure exerted will be 0 isn't it. So, reflection coefficient is for determining that how much percentage of the solute can cross the membrane. So, if it is not able to cross the membrane at all then this reflection coefficient will be 1 and if it is crossing the membrane freely right. So, then that reflection coefficient will become 0 and the osmotic pressure exerted by such ineffective osmol will be 0. Let us come to next part that is tonicity of the solution tonicity of the solution. So, normal plasma tonicity is 280 to 290 milli osmoles per litter and when we consider another solution we have to see what is its osmolarity with respect to the plasma osmolarity ok. So, isotonic solution is something which has the same tonicity as that of the plasma, hyper tonic solution will have the osmolarity more than that of the plasma. So, maybe 300 milli osmoles per litter that is a hyper tonic solution and hyper tonic solution is something which is having osmolarity less than that of the plasma osmolarity. So, these terms isotonic, hyper tonic and hyper tonic are always used with respect to tonicity of the plasma. And what are the examples of such kind of solution? Well, 0.9 percent NaCl is a isotonic solution and one good exercise will be to calculate the milli osmos from this 0.9 percent solution isn't it? And then there is 5 percent glucose. Actually this 5 percent glucose if we see the osmolarity then it is similar to that of the osmolarity. However, glucose actually can be metabolized and after metabolism what happens that only water will be left isn't it? So, when the solution is kept separate from the body ok then the tonicity is same. So, it is iso osmotic, but once it is injected in the body what happens that this glucose is metabolized ok. So, the physiological effect is not like injection of a isotonic substance instead it behaves like a hypotonic solution ok, hypotonic solution because glucose will be metabolized. So, please try to understand the difference between iso osmotic and isotonic. 0.9 percent NaCl is both iso osmotic and isotonic to that of the plasma, but 5 percent glucose is iso osmotic, but it is hypotonic ok. With this now let us solve certain MCQs for these concepts. So, first MCQ is contribution of plasma proteins to osmolarity osmolarity is less because. So, there is high concentration of proteins that is correct high concentration of proteins is there, but contribution to osmolarity is less why so? Because you see the molecular weight of proteins is much higher very high we were talking about ions and all right whose molecular weight is much less plasma protein molecular weight is much higher. So, with the same grams if we talk about right with the same weight or grams the number of osmols which will be there will be very less understanding for the same weight of the ions the number of osmols will be very high. So, what is the answer in this case? So, first obviously it is high molecular weight high molecular weight is there. So, these two we rule out A and C what about B and D? The molar concentration the molar concentration actually is low ok. So, molar concentration is low and hence osmols will also be low. So, this is the correct answer low molar concentration and high molecular weight. Coming to next question calculate osmolarity of a solution containing 20 millimoles of NaCl, 5 millimoles of KCl and 5 millimoles of CaCl2. So, it is simple what you have to do is calculate the osmols of this. So, 20 millimoles of NaCl how many osmols will be there? How to calculate the osmols? We have to multiply it with the number of particles which it can release isn't it? So, NaCl can release two particles. So, 20 into 2 it will be 40 milliosmoles ok. Then 5 millimoles KCl again two particles it can release. So, it will be 10 milliosmoles right and what about CaCl2? It can release three particles. So, it will be 5, 3 is a 15. So, it will be 15 milliosmoles and what will be the answer of this? 65 milliosmoles are present in this solution ok fine. Let's move on to next question. What will happen to red cell volume when they are placed in a solution of 140 millimoles NaCl plus 20 millimoles of urea and this red cell volume if you see the osmolarity is 280 milliosmoles per liter. So, red cell is having a osmolarity of 280 milliosmoles per liter and here we have 140 millimoles of NaCl right and plus 20 millimoles of urea. So, what is the osmolarity of this solution? See 140 millimoles of NaCl is equivalent to 280 milliosmoles of NaCl right. So, actually this becomes same, but it has 20 millimoles extra urea also. So, actually this solution is hyper osmotic, hyper osmotic. What about tonicity? So, always solve this way. First label it as hyper osmotic and then find out whether it is what is its tonicity with respect to the cell or plasma right. So, it is hyper tonic fine. So, where will the water go? It water moves from low solute concentration to high solute concentration. So, water will move from RBC to that of the solution. So, initially cells will shrink, initially they will shrink right and then what will happen this 20 milliosmoles actually of this urea, it will equilibrate because urea can cross the cell membrane ok. So, if urea crosses what will happen this? It will be 10 millimoles here and 10 millimoles here. So, it will be 20, 290 and 290 milliosmoles. So, because it takes some time to equilibrate that is why I said that initially they will shrink, but what will be the end result? See because the osmolarity both sides it is same since the solute can cross. So, ultimately there will be no change in cell volume. We cannot say that they will swell understanding because the osmolarity is same on both sides. If it was like 280 here and whole urea it was becoming coming inside right then this osmolarity becomes more and more water will move. But if the osmolarity on both sides is same then there will be no next movement of the water. So, even if initially they shrink ok maybe, but later on they will come back to their original volume and they will not swell. So, basically the answer is no change in cell volume ok. Let us come to next question. Osmolarity determined by freezing point depression and by formula were respectively this. So, this 320 is by freezing point depression and 290 milliosmoles is by the formula. So, what could not be the cause of this? See not be the cause we are asking. So, it is simple that the answer cannot be hyperglycemia. Why? Because we have already included glucose in the formula right glucose ions and blood urea nitrogen is already in the formula so, if glucose concentration rises it will reflect as rise in osmolarity by the formula as well and by the freezing point depression as well isn't it? So, there will not be much difference between the two it will be coming ok only. But the other substances that is these substances they are not included in the formula. So, if they are not included in the formula that means they will actually cause the freezing point depression much more than what is expected. So, the osmolarity determined by this will be higher ok and these are not reflected in the formula. So, the difference between the two methods will be more.