Hey everyone welcome to homeschool and we have started class 11 chemistry series where we have already started a first chapter that is basic concepts of chemistry actual chemistry topics started from previous videos onwards I mean from the loss of chemical combination which we have already discussed in the last video Links are provided in the description. You can go and check with the links. See the real topic actually begins from today that is from this video onwards.
Since you have selected science stream keeping the aim of competitive examinations, you should thoroughly have a clarity on certain concepts because those concepts are really important for NEET, JEE, etc. So the topic I selected for today's class is atomic mass and molecular mass. See you must clearly understand that in-depth meanings of atomic mass and molecular mass. This is really very important to understand okay to understand the so-called mole concept. Mole concept and stoichiometry which you will study next I mean in the next video are the most important topics of class 11 and 12. And definitely one question or two questions would surely come for competitive exams from this particular topic.
So the today's topic is very very very important to understand carefully. The topic is atomic mass and molecular mass. There are very much different meanings behind these words atomic mass and molecular mass.
First let us try to discuss about so called atomic mass. So what do you mean by atomic mass? The word is not new for us. Atomic mass word is not new for us but its deep meaning is very much new for us that I am going to tell you in few minutes.
So atomic mass. See the moment I say atomic mass generally the thing that can flash into our mind is it is nothing but mass of one atom of an element. Mass of one atom of an element. Okay. So if you take up any element, say you take sodium element, a piece of sodium, it will have crores and crores of atoms, right?
And one atoms of sodium will have crores and crores of atoms. Mass is called as atomic mass. Okay. For example, for example, whenever I ask you to tell atomic mass of oxygen.
Okay. What is your answer? Atomic mass of oxygen is 16. You would quickly say 16. Okay.
And do you remember some unit that we will put for atomic mass? That is AMU. We will write the unit as AMU. or we can also write it as 16u right.
Say about these things you have studied in your previous classes I mean in your lower classes. Anyway I am going to tell you what do you mean by this amu or u okay. So as of now just try to keep it in a mind that amu is a sort of unit not actually the unit but a kind of unit that we will use for atomic mass. Say atomic mass of oxygen is 16. When I ask you atomic mass of nitrogen, you would quickly say 14, right?
So atomic mass of nitrogen is 14 amu, I can write. right so in a similar way atomic mass of hydrogen when i ask you you would say one atomic mass of carbon when i ask you you would say 12 like that we all know the concept of atomic masses in a periodic table it will be mentioned right and you know these atomic masses really have a different meaning actually say generally to say atomic mass is nothing but mass of one atom right okay you'll have a certain digit it was already found by the scientist okay every elements atomic mass was already found by the scientist and they have put their values and it it has been mentioned in the periodic table we all know this but how did the scientists find this atomic mass you know why why they gave 16 as the atomic mass for oxygen why they gave as 14 as the atomic mass of nitrogen you know there is a lot of work done uh before inventing this digits i mean before inventing this atomic mass okay so we all know atomic mass is generally nothing but mass of one atom keep this in point mass of one atom Okay. And now I'm going to tell you how they got these values, how they found these atomic mass values for different elements.
Okay. See, actually speaking, nowadays, nowadays, we have a well sophisticated instruments. There is an instrument called spectroscopy. I repeat once again, spectroscopy. So using this spectroscopy, one can find atomic mass of any element.
Okay. So when you load a element into a instrument its atomic mass value that is mass of one atom's value is displayed in the computer monitor right so it has become such so much easy it has become very much easy nowadays to find the atomic mass but long back long back you know there was no such well advanced instruments to find the atomic masses during those days scientists took a reference element to find the atomic masses of other elements existed in a nature. So what did scientists do? Scientists have taken a reference element. So atomic mass can be found with the help of reference element.
So first point I am writing on a board is note down atomic masses Atomic masses, whatever we are using now and whatever it has been displayed in the periodic table by the scientist, those atomic masses, okay, were found, were found by taking, by taking carbon-12 isotope. This is very, very important point. Carbon-12 isotope as a reference.
as a reference okay say they took carbon 12 isotope as a reference so here how they took it as a reference and how they found everything i'll tell you but now the first question is why did they take carbon 12 isotope as a reference. Okay. And first try to understand what do you mean by this carbon 12 isotope.
You know there are three isotopes for carbon. There are how many isotopes? There are three isotopes for carbon.
Three isotopes are present for carbon. They are carbon 12, carbon 13 and carbon 14. So these are the three isotopes. Isotopes means atoms of the element having same atomic number but different atomic masses. So here carbon 12 isotope is abundantly available in our nature.
It is available to a larger extent and it is the most stable isotope of carbon. That's the reason carbon-12 isotope was taken as a standard. So, what is the reason here?
Reason. Take down. The reason for taking carbon-12 isotope as standard, as reference.
Okay. So, what are the two reasons? There are two reasons. The first reason is carbon-12 isotope.
is abundantly available in our nature it is abundantly available okay in nature clear and the second most important reason is it is Most stable isotope. It is the most stable isotope of carbon. Okay. So these are the two reasons why they took carbon 12 isotope as reference.
And now what did they do with this reference? You know. They just compared every other elements mass with the mass of carbon 12 isotope.
They did not take full atom of carbon 12 as a standard. They took just. 1 by 12th part of carbon 12 atom.
Okay. They took the mass of 1 by 12th part of carbon 12 atom as the reference. For example, if this is a carbon atom, divide this carbon atom into 12 parts. Okay.
Let me divide. So, this is one part. two parts three parts four parts right five parts six parts seven eight nine right?
10, 11, 12. Okay. So this is how I divided carbon 12 atom into 12 parts. And you know, consider the 12th part, any one part, like consider the 12th part of carbon. Okay.
Any one part in 12 parts, you consider this part, right? And they somehow found, theoretically, they found the mass of, the mass of 1 by 12th part. part of carbon atom.
Okay. So they found 1 by 12. This one part's mass was found. Okay.
So that mass was found to be 1.67 into 10 to the power of minus 24 grams. Okay. So this is actually the mass of mass of 1 by 12th part.
of carbon 12 atom okay so what is this value 1.67 into 10 to the power of minus 24 this value is very important so what is this it is the mass of 1 by 12th part of carbon 12 atom Okay. So single atom divided into 12 parts. So one part's mass was found. How much?
1.67 into 10 to the power of minus 24 grams. And that is the value which is nothing but mass of 1 by 12th part of carbon 12 atom. Okay. So this was taken as a reference.
This was taken as a reference. Okay. So by taking this As a reference, they found atomic masses of every other elements.
Okay. Say now I'm going to give you the actual definition of atomic mass. So try to understand the definition first and you will get an idea how they found atomic masses of other elements. elements by taking this part, this 1 by 12th part of carbon atom as a reference. Okay, so just try to observe the definition here.
Very, very important definition. You can note it down in your book. Atomic mass is a number. It is just a number which indicates, which indicates Very very important definition.
Atomic mass is a number which indicates, listen carefully, how many times single atom of an element is heavier than. is heavier than, than mass of 1 twelfth part of, part of carbon 12. atom okay so how many times the single atom of other element is heavier than the mass of 1 by 12th part of carbon atom okay say for example if this is 1 by 12th part of carbon okay if this small chalk piece is 1 by 12th part 1 by 12th part of carbon atom okay it has got some mass it has got some weight okay i know its value i know its value okay say let me take its value as 1 gram for simplicity i am taking its value as 1 gram okay and i want to find out the mass of this chalk piece i know the mass of this small chalk piece its mass is one gram okay and i want to find out the mass of this particular chalk pencil along with the chalk piece i don't have weighing machine okay so what i would do here I will compare the weight of this guy in comparison to this guy. I know its mass, 1 gram. Okay.
And I'm holding this in one hand and I'm holding this in another hand. And I am feeling that this guy's mass is 10 times heavier than this chalk piece. I can feel it.
I can feel it. See, in one hand, small chalk piece is there. In another hand, this big chalk pencil is there.
And I can feel that this guy is 10 times heavier than this small chalk piece. Okay, if I know the mass of this small chalk piece as 1 gram, then what will be the mass of this chalk pencil? I am telling it is 10 times heavier.
That means 1 into 10, 10 grams. So that way I can find out the mass of unknown things in comparison to the reference. So here the small chalk piece is a reference.
I know its mass and I am comparing unknown substances mass. Okay, say by comparing. Right? Say that is what they have done here. Okay?
Say imagine this is one twelfth part of carbon atom and this is single atom of other element. Okay? And I am comparing.
If it is 16 times heavier than this guy, then its atomic mass is 16. 16 into mass of this one. Mass of this one. Okay?
Clear everybody? Got to know? So if this is the mass of 112, this is 112th part of carbon atom, I am comparing every other element with this small piece, okay? If this guy is two times heavier than this one, then it's atomic.
mass is 2 okay if this guy is 10 times heavier than this then its atomic mass is 10 it's like it is just a number that indicates how many times the atom of other element is heavier than this 1 12th part of carbon atom okay so that number we will call it as atomic mass clear Okay, so definition is very very important to understand, right? Okay, and now I gave you mass of one twelfth part of carbon atom is 1.67 into 10 to the power of minus 24 grams. okay and this value this value itself we will call it as amu this is what we will call it as 1 amu okay so this value is called 1 amu amu means atomic mass unit what is amu atomic mass unit okay so if i have to Find out the atomic mass of oxygen.
I will compare oxygen atom with this particular mass. Okay. How many times oxygen atom is heavier than this value?
That is the atomic mass of oxygen atom. Okay. So we have written atomic mass of oxygen is 16 AMU. Do you know what is the meaning of 16 AMU? Okay.
AMU means what? 1.67 into 10 to the power of minus 24. That means our single oxygen atom is 16 times heavier than this value. What is this value?
It is the mass of 1 twelfth part of carbon 12 atom. Okay, so our single oxygen atom is 16 times heavier than mass of one twelfth part of carbon atom. That is why we will write 16 AMU.
So what is AMU here? AMU is nothing but this much gram. Okay, so AMU has that much meaning.
AMU means mass of one twelfth part of carbon 12 atom. Okay, so you must know the definition of 1 AMU. What is AMU? It is the mass of 1 twelfth part of carbon 12 atom.
What is its value? 1.67 into 10 power minus 24 grams. Okay, so what is the meaning of atomic mass of oxygen is 16 AMU? It means oxygen atom.
Oxygen atom is 16 times heavier than 1 twelfth part of carbon-12 atom. Okay, so oxygen is how many times heavier than 1 twelfth part of carbon 12 atom? It is 16 times. This meaning is very very important to understand. So our oxygen atom is 16 times heavier than 1 twelfth part of carbon 12 atom.
Okay, so this is the real meaning of atomic mass. Fine. So now you are going to tell me the answer for me. Whenever I say atomic mass of sodium, sodium is 23 amu.
What is the meaning? Say its meaning you are going to write in a comment section. Okay.
Say I will be waiting and I will definitely mention your name in the next video that who has mentioned the answer first in the comment section. Okay. Definitely I will mention. uh the student's name in the next video right so write the answer right now you know for this particular sentence so what is the meaning of atomic mass of sodium is 23 amu i i told you the meaning atomic mass of oxygen is 16 means what oxygen atom single oxygen atom is 16 times heavier than 1 twelfth part of carbon 12 atom. Say one more example I can tell you.
Atomic mass of nitrogen is 14 means what? Single nitrogen atom is how many times heavier than 1 by 12th part of carbon atom? 14 times heavier.
14 times heavier than 1 by 12th part of carbon atom. Okay. Say similarly, you are going to write the actual meaning for 23 AMU.
Atomic mass of sodium is 23 we say. It is 23 AMU. That means what?
So quickly write the answer in the comments and come back to the video. Fine. So now that we understood about atomic masses.
See every element's atomic mass was found like this only that is with respect to 1 12th part of carbon 12 atom. And it's very important to remember that value. Okay. So what is the mass of 1 twelfth part of carbon 12 atom? It is 1.67 into 10 power minus 24 grams which is what we will call it as AMU.
That value is called as AMU. Okay. Fine. Similarly we will discuss molecular mass. Similarly we will discuss molecular mass.
Okay. So the same meaning say let me take a molecule. You know, let me take an example of water.
Molecular mass of water. water. Can you tell me what is the molecular mass of water? It is 18 amu. We will write it as 18 amu.
Okay, so what is the meaning of this 18 amu? That means one molecule of water. I am writing the meaning.
One molecule of water is 18 times heavier than than 1 by 12th part of carbon 12 atom okay carbon 12 isotopic atom okay say this is what the real meaning of molecular mass also and practically how do you calculate molecular mass you know molecular mass is nothing but molecular mass is the sum of atomic masses Atomic masses of elements. Elements present in a molecule. Present in a molecule is called molecular mass.
Okay. So for example water is there no. There are two atoms. Hydrogen atom and oxygen atom.
Right. So hydrogen atoms mass is atomic mass of hydrogen is 1. How many hydrogens are there? 2. Maldives.
with 2 plus there is another element another type of element called oxygen. So what is the atomic mass of oxygen here? 16. So 16 plus 2 you will have 18. So like that practically we can calculate molecular mass of any molecule. Okay practically if you want to calculate in a molecule you will have certain atoms no? You will have certain elements their atomic masses you should add.
Okay, so when you add the atomic masses, you will get molecular mass. But actual meaning of molecular mass is this only. See molecular mass of water 18 I told you.
So what do you mean by this 18? It is 18 times heavier than 1 by 12th part of carbon 12 isotopic atom. Okay, say and now let us work it on the molecular masses. Say in chemistry chapters molecular mass calculation is very important because its value is very much important for certain calculations. So when a molecule is given you must know how to calculate their molecular masses.
Okay and now let us learn how to calculate molecular masses for some compounds. Say carbon dioxide. What is the molecular mass of carbon dioxide? Say how do you calculate it practically? You just have to do sum of atomic masses of all elements present in a molecule.
Right? So what are the elements present? Carbon and oxygen.
They are atomic masses. You should add. That's it. So what is the atomic mass of carbon?
12. Plus what is the atomic mass of oxygen? 16. But how many number of oxygens are there? 2. So don't forget to multiply with number of atoms.
So it is 12 plus you know 32. So you will have the molecular mass as 42 AMU or 42 U. AMU or U. You can mention it as a unit. But what is the meaning of 42 U? The actual meaning is one molecule of carbon dioxide is 42 times heavier than 1 by 12th part of carbon 12 isotopic atom.
Okay? One molecule of carbon dioxide is 44 times, sorry it is not 42, 44 times heavier than 1 by 12th part of carbon 12 atom. Okay?
So that is the real meaning but practically we can calculate just by adding the atomic masses of all elements. Okay? And similarly can you calculate for C6H12O6?
Say same thing, atomic mass of carbon 12. How many carbon atoms are there? 6. So multiply with 6. Plus atomic mass of hydrogen 1. How many hydrogen atoms are there? 12. Plus atomic mass of oxygen 60. How many number of oxygen atoms are there? 6. Okay.
So when you add all these values, you know what is the answer you will get? You will get it as 180. right? So 180 is the molecular mass of sugar that is C6H12O6.
Similarly okay say post your answer in the comments and this is a homework question for you. Atomic mass of C2H5OH okay? So what is the atomic mass of ethyl alcohol C2H5OH?
So quickly post the answer in the comment section. So this is all about molecular mass. Finding molecular mass is really a very very important concept.
So today we have learnt about atomic masses and molecular masses. That's all about today's session. And in the next session I will come up with average atomic mass calculation.
That's also really an important topic for board examination point of view actually. and let us see the importance of average atomic mass why we have to calculate what is the formula everything we'll discuss and after that comes your mole concept and stoichiometry and every aspect I will cover by keeping competitive examinations in our mind because our main focus is to crack NEET or JEE with a better rank right to get a seat in the medical colleges as well as engineering colleges and you know what board examinations are much easier just like your 10th grade 12th 11th board examinations are pretty much easier cracking competitive examinations is the big challenge for you anyway you know the entire syllabus must be understood thoroughly and you should able to apply the concepts when a question is asked in a different way that is the skill necessary to crack the competitive examinations so after the concept is covered i will show you few competitive questions also so that you will get an idea how to solve a question and along with the questions i will show you different tricks shortcuts you know how to save the time in the competitive exams with little bit of tricks right all that we will discuss so actual concept just now it has started i mean from last video it has started and in the next video along with the calculation of average atomic mass we will discuss few competitive questions uh whatever is over so far okay thank you so much and take care let us meet up