this last video of chapter 2 will cover the second half of section 4. we'll see some important units we use in chemistry and learn how to perform some calculations to convert quantities pause to read the learning objectives did you know that the number of molecules in a single droplet of water is roughly a hundred billion times greater than the number of people on earth in chemistry we study very small things in very big quantities because of that we use some units that you might not be familiar with yet the mole the mole is a unit that corresponds to the amount of substance in the international system of units it is defined as the same number of atoms in a 12 gram sample of pure carbon 12. it is similar to units like pear and dozen if you have a dozen donuts you have 12 donuts and if you have 12 eggs that's a dozen eggs the mole connects the mass of a sample and the number of atoms molecules or ions in that sample it is a very large number that's the number in scientific notation it is 6.022 times 10 to the power of 23. this constant is named after amedeo avogadro an italian scientist one mole of any substance contains 6.022 times 10 to the 23rd particles that's the number of atoms in each of these samples while one mole always have the same count of particles the masses vary depending on the chemical composition of the substance since atoms of different elements have different masses while one mole of carbon only weighs 12 grams one mole of lead weighs 207 grams we define as molar mass the mass in grams of one mole of that substance the unit of molar mass is grams per mole since both the mole and the atomic mass unit amu are based on carbon-12 as the reference substance the molar mass of any atom is numerically equivalent to its atomic mass in amu for instance one atom of carbon 12 has a mass of 12 amu one mole of carbon-12 atoms have a mass of 12 grams if one water molecule has a mass of 18 amu one mole of water molecules have a mass of 18 grams all samples in this picture contains one mole of molecules or formula units or 6.02 times 10 to the 23rd note how the appearance and the mass very drastically between these samples i'm now going to show you how to perform some calculations to convert between various quantities that describe the composition of substances and compounds by using formula masses or molar masses the mole and avogadro's number to convert between number of particles and moles we use avogadro's number and to convert between moles and mass we use the molemets example one according to nutritional guidelines from the u.s department of agriculture the estimated average requirement for dietary potassium is 4.7 what is the estimated average requirement of potassium in moles the first step we take is to identify the information provided by the problem in this case the mass of potassium in grams which is 4.7 next we have to find out what we want to calculate in this case the average requirement of potassium in moles so we want to calculate moles of potassium atoms now we're going to plug in the numbers first what we have 4.7 grams of potassium now to convert between grams and moles we're going to use the molar mass in grams per moles we need moles to be on top and grams on the denominator in order to be able to cancel out the unit grams so that our final venture will be in moles and that is point 12 moles of potassium so now we know that to convert between grams and moles we need to divide by the molar mass example two a litter of air contains 9.2 times 10 to the negative fourth moles of argon what is the mass of argon in a liter of air so first step what do we have moles of argon atoms what do we want mass of argon atoms so we start from what we have 9.2 times 10 to the negative 4 moles of argon we're going to multiply by a conversion factor between moles and grams that's the molar mass we want now moles to be on the denominator so that we can cross that out so now applying these numbers in your calculator we you get 0.037 grams of argon so from moles to grams you need to multiply by the molar mass in grams per mole you don't have to memorize anything you just have to look into the units and make make sure that they cancel each other and that at the end of the day you have the unit you're looking for example three copper is commonly used to fabricate electrical wire how many copper atoms are in five grams of copper wire the molar mass of copper is 63.55 grams per mole why don't you pause this video and try to solve this on your own before all right so we have here the mass of copper atom in grams as five grams given by the problem we want to calculate the number of copper atoms that's what the question is asking for we can't go directly from grams to atoms we need to pass through moles moles is always the unit that links mass in particles so here we start from the mass we were given 5 grams of copper we're going to use the molar mass so one mole of copper contains 63.55 grams so that we can cross out grams now from moles we're going to convert into atoms using avogadro's number so 6.02 times 10 to the 23rd atoms per mole we could cross out moles and our result is going to be given in atoms so we plug these numbers in the calculator we get 4.74 times 10 to the 22nd atoms of copper to get from grams to moles we divided by the molar mass to get from moles to number of atoms we multiply it by avogadro's number example four our bodies synthesize protein from amino acids one of these amino acids is glycine which has the molecular formula c2 h5 o2 n how many moles of glycine molecules are contained in 28.35 grams of glycine first what the problem gives us the mass of glycine that then what the problem wants us to calculate how many moles of glycine so from grams to moles we're going to divide by the molar mass in grams per mole we weren't given we first have to calculate the molar mass for glycine in order to do that we need to figure out how many atoms each of each type the molecule has so from the molecular formula we know that glycine has two carbons five hydrogens two oxygens and one nitrogen one mole of molecule is going to have that same number of moles of each element so two moles of carbon five moles of hydrogen two moles of oxygen and one mole of nitrogen now the molar mass are gonna always be given that's the information you can find in the periodic table so here is um here are the numbers now in order to get the contribution of each element to the molecular mass the molar mass we're gonna multiply the quantity by the molar mass for each element and here you can see the numbers then we add up all these numbers and we find that the molecular mass or molar mass of glycine is 75.07 that's in grams per moles all right so the mass of glycine that the problem gave us was 28.35 grams now we use the molar mass we want grams to be in the denominator so that we cross out and then we can get to 0.378 moles of glycine your textbook have other problems if you want to look at them