 Let's go through this problem which says that 50 ml of 0.1 molar HCl is added to 25 ml of an NH3 solution. 21.5 ml of 0.1 molar NaOH was required to neutralize the excess HCl. So what is the concentration of the ammonia solution? So you can pause the video here and try it out and then we'll continue. So let's start by writing down information that is given in the problem and let's try to understand what the problem is asking us to calculate. So first we were given that the reaction is between 50 ml of 0.1 molar HCl and we don't know the concentration of ammonia but we are told that the volume of the ammonia is 25 ml. So if we write down the reaction, we know that the product form will be NH4Cl. Now we are also told that there was some excess HCl. So what this means is all of the ammonia that was used in this reaction was fully consumed and there was some HCl left and then we are given one additional piece of information. We are told that whatever was the amount of HCl that was in excess, that reacts with 21.5 ml of 0.1 molar NaOH and we get complete neutralization. So now using this information, how can we calculate the concentration of NH3? So if we start with this reaction, we know that all of this 25 ml of ammonia was consumed in this reaction. So if you want to calculate the concentration of ammonia, we need the volume of ammonia which is given here and we need to know the number of moles of ammonia that were consumed in this reaction. So how can we find the number of moles of ammonia consumed? Well, we know that the mole ratio of HCl to NH3 is 1 is to 1 because this is a balanced reaction and so the number of moles of ammonia consumed will be equal to the number of moles of HCl that is consumed in this reaction. But we don't know how much of HCl was consumed in this reaction. We only know what was the starting number of moles that we can calculate from here. So then how do we find out how much was the number of moles of HCl consumed? For that we can use this reaction. Now since we know that whatever was the excess amount was totally neutralized by this 21.5 ml of 0.1 molar NaOH and since the mole ratio here is 1 is to 1, we can calculate the number of moles of NaOH consumed which will give us the number of excess moles of HCl and then we know that the total number of moles that we started with will be equal to the number of moles consumed here plus the number of moles in excess. So from the second reaction we can calculate the number of excess moles of HCl and from this information we can calculate the initial number of moles. So by using this relation we can calculate the number of moles of HCl consumed and we know that the number of moles of HCl consumed will be equal to the number of moles of NH3 consumed which we can use to calculate the concentration of NH3. So let's start by calculating the initial number of moles. For that we will use the formula for molarity which is equal to the number of moles divided by the volume in liters. So, we know the volume and we know the molarity which was for the initial solution. So, we can write down that 0.1 will be equal to the number of moles of HCl initially present divided by the initial volume which is 50 ml or 0.05 liters. So, now if you solve this we find the initial number of moles of HCl is 0.005 and similarly since we know that the mole ratio here is 1 is to 1 the number of excess moles of HCl will be equal to the number of moles of NaOH and again we have the molarity here which is 0.1 and we know the volume in liters which is 0.0215 liters. So, from here we can also calculate the number of moles of NaOH which will be equal to the number of moles of HCl. So, if we do that we can write the number of moles of HCl or the number of moles of NaOH divided by the volume which is 0.0215 will be equal to the molarity which is 0.1. So, if you solve this you get the number of moles of excess HCl to be 0.00215. Now that we have the initial number of moles and the excess number of moles we can calculate the number of moles of HCl that is consumed as 0.005 minus 0.00215 which we calculate from here. So, the number of moles of HCl that are consumed is 0.00285. So, now we know how many moles of HCl were consumed in this reaction and as we discussed because the mole ratio is 1 is to 1 the number of moles of HCl consumed will be equal to the number of moles of ammonia consumed. Now, using this information let's calculate the concentration of the molarity of this ammonia. So, we know that molarity will be equal to number of moles divided by the volume in liters and in the previous step we calculated the number of moles of NH3 consumed and the volume is given here. So, we can calculate the molarity as 0.00285 from our earlier step divided by 0.025 which is the volume in liters and if we solve this we get the molarity or the concentration to be equal to 0.114 moles per liter.