The MoleJun 09, 2021
chemistry lecture number 51 el
molemany times we use a word to describe a numerical quantity, for example, we will say a dozen eggs instead of twelve eggs and it is easier to say a ream of paper than to say 500 sheets of paper in the same way In chemistry we use the
moleto represent a specific number in the same way that the word dozen means 12 and the word ream means 500. The word mole sometimes abbreviated to mo L represents the number 6.02 multiplied by 10 to the power of 23, so one mole is this number 6.02. multiplied by 10 to 23 is used to describe representative particles and what our representative particles are a representative particle is a molecule of atom or a formula unit of an ionic compound, so helium, well, it is an atom of h2o, it is a molecule of NaCl, is an ionic compound, we refer to this.
As a form of unit, moles are used to describe the number of atoms, molecules or formula units we have now, where did this number 6.02 times 10 to the 23rd power come from? Well, it was defined as the number of atoms in 12 grams of carbon. 12 now carbon-12 is an isotope of carbon. You remember this notation, so this is a carbon atom that has six protons and then add another six and that will give you the total number of neutrons, so carbon-12 is a particular type or isotope. of carbon that is one that has six protons and six neutrons another carbon isotope is carbon-14 that has six protons and eight neutrons but we are only interested in carbon-12 anyway if you have 12 grams of this carbon-12 isotope you have one mole or 6.02 times 10 to the power of 23 carbon-12 atoms, which means that twelve grams of carbon-12 will have this many particles or this many atoms, so twelve grams of carbon-12 will have that many of carbon atoms.
Okay, so 6.02 times 10 to the 23rd power, it's also called Avogadro's number, okay, so we'll convert particles to moles and vice versa, we'll calculate the number of moles that contain one point two oh four times ten to twenty-four sodium atoms. . How many moles represent so many sodium atoms? This is what we are going to do: we will convert atoms to moles and to do this we need to use a certain relationship. We need to know that one mole of sodium means you have 6.02 times. 10 to the power of 23 sodium atoms, okay, so we're going to label some factors to convert this amount to moles, so when we factor the label we take the number we want to convert and write it as a fraction 1.20 4 times 10 raised to 24 atoms. of sodium and we'll convert it to a fraction by putting it over 1, so that's what we want to convert, we want to convert it from atoms to moles.
Well, I have atoms on me and I want to get rid of them. I need to put atoms at the bottom, I'll put 6.02 times 10 to the 23rd atoms at the bottom and 1 mole at the top, so 6.02 times 10 to the 23rd sodium atoms and then one mole sodium on top, all right, so sodium atoms on top. will cancel out with sodium atoms at the bottom 1.20 4 times 10 to the power of 24 divided by 6.02 times 10 to the power of 23 that will give your calculator we will tell you: we need the correct number of significant digits, these are three digits significant digits, that's four significant digits we take the least number of significant digits, so we'll call it two dot zero zero times 10 to the no, it's just going to be two, sorry, yeah, two moles of na, so what that means is that If you have that many sodium atoms, you have two moles of sodium, okay, let's try another one, it's similar, calculate the number of moles that contain four point five zero times ten to twenty-four zinc atoms, very good, once again, we are starting with the atoms we want to convert. in moles, so we're going to convert atoms into moles and we need an equality.
One mole of zinc is equal to 6.02 times 10 to the power of 23 zinc atoms. Now we take the number we want to convert and write it as a fraction. or 0.5 0 times 10 to the 24th power of zinc atoms we are trying to go from atoms to zinc. Sorry, atoms to moles. I have atoms on me. I want to get rid of it, so I'm going to put 6.02 times. 10 to the 23 atoms at the bottom and then 1 mole at the top, all the right atoms at the top will cancel out with the atoms at the bottom, leaving us with moles of zinc at the top, which is what what we want, so 4.5 times 10 to the power of 24 divided by 6.02 times. 10 to 23 let's go ahead and do that 4.5 zero times 10 to 24 / 6.02 times 10 to 23, okay, so that's our raw answer seven point four seven five seven point four seven five zero eight three and then we want three significant digits because that is the smallest number of significant digits in our two operational numbers this number happened here this is greater than five it is not five it is 508 three which is greater than just five so we round this will be seven point four eight moles of zinc, okay , let's try another one, how many molecules of sucrose are there in 3.5 moles of c12 h22 o11, so that is the formula of sucrose, here we go, let's see, they give us moles 3.5 moles and ask how many molecules are we going to convert to moles in molecules I know that one mole of sucrose is the same as 6.02 times 10 to 23 molecules of sucrose, okay, so let's write the number we want to convert as a fraction three point five zero moles c12h22o11 say it about one write as a fraction, okay, we want to go from moles to molecules.
I have moles of sucrose on me. I want to get rid of it, so I need moles of sucrose at the bottom. So I'll write one mole of sucrose on top. the bottom and so many sucrose molecules on the top, so one mole c12h22o11 is the same as 6.02 times 10 to the power of 23 c12h22o11 molecules, okay, we have moles of sucrose on the top, the moles of sucrose at the bottom, we are left with sucrose molecules. which is what we want our final answer 3.5 0 times 6 times 2 times 10 to the power of 23, let's see if we get three point five zero times 6.02 multiplied by 10 to the power of 23, that will give me a raw answer to point one zero seven times 10 to 24 we want to round this to three significant digits because we have three significant digits here and here, so it's seven, which is greater than five, so it's going to be two point one one times 10 to the power of the molecules 24 c12h22o11, okay. , you can do one more.
How many formula units of silver nitrate are there in three point two five moles of silver nitrate? Okay, let's look here again, let's go three, let's go from moles to formula units. Okay, and I know that one mole uses the same as 6.02 times 10 to the power of 23 to me the units should be approximately that first, okay, so three point two five moles of silver nitrate, that's the amount we want to convert, put that about one, we have moles of silver nitrate on top, I want to get rid of So I need to put moles of silver nitrate on the bottom, so I'm going to put this on the bottom, one mole of silver nitrate and then on top of that one mole of nitrate of silver is the equivalent of 6.02 times 10 raised to the 23 formula units of silver. nitrate, okay, moles of silver nitrate on the top, moles of silver nitrate on the bottom, cancel three point two five times six binary two times ten to the 23rd three point two five times 6.02 times 10 to the 23, okay, that's going to give me raw answer one point nine five six five times ten to the power of 24 and we want three significant digits, so this will be rounded to 1.96 times 10 to the power of 24 units formula AG + o3 very okay, many factor labels in this section here for a PDF transcript of this lecture go to wwe.com this has been the 51st chemistry lecture the mole
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