form-2220.com is not affiliated with IRS

Here we're going to learn how to solve problems like these we're given the empirical formula and the molar mass of a compound and we have to figure out that compounds molecular formula we're asked to determine the molecular formula of a compound with the empirical formula CF 2 in a molar mass of 200 point 0 4 grams per mole let's begin by making a chart ok so here's a formula the empirical formula that we're starting with and I want to find its molar mass so to do that I'll look up carbon and fluorine on the periodic table I do 1 times the molar mass of carbon plus 2 times the molar mass of fluorine and I get 50 point zero one okay now CF 2 this here is an empirical formula which means that the number of atoms here the ratio of atoms here is a simplified or reduced as possible now there are a number of different molecular formulas that all have CF - is there empirical formula ok so for example C 2 F 4 is a molecular formula that has this as its empirical formula okay if we took both of these numbers and divided them by 2 we'd end up with CF 2 and going the other way we can kind of say that we got this formula by taking the empirical formula and multiplying it by 2 in other words this here this formula is kind of a multiple of the empirical formula we multiply it by 2 and we get this now it's the same for the molar mass okay we take the empirical formula multiplied by 2 and get this to find the molar mass of C 2 F 4 we can just take the molar mass of the empirical formula multiply it by 2 and get the molar mass of C 2 F 4 100 point 0 2 twice what this is here's another formula that has CF 2 as its empirical formula C 3 F 6 we get by multiplying empirical formula by three and the same thing for the molar mass we can figure out the molar mass of this compound by taking the molar mass the empirical formula multiplying it by three and getting 150 point zero three we could also take the empirical formula multiply it by four you're probably getting the point here we get C 4 F 8 is a molecular formula and it's molar mass 50 point zero one times four is 200 point zero four this is the molar mass that we're looking for so it means that this C 4 f 8 is the molecular formula that we want so what we're really asking is what do we have to multiply the empirical formula by to get a molecular formula that has the molar mass that we need that's the background thinking that we want to do with a problem like this I think this chart really lays things out really clearly and turn it in terms of what's going on with the multiples here but I bet you don't want to have to make a chart like this every single time you solve one of these problems okay so now that we understand the background of this I want to show you a simpler way that we can go through solving problems like these now as we said before problems like this are really all about finding what number we want to multiply the empirical formula by to find the molecular formula that has the right molar mass okay we want to find this number to multiply the empirical formula by here's one way we can get okay we can use this equation here where we take the molar mass of the molecular formula so in this case it's going to be 200 point zero four we get that from right up there and then we divide by the molar mass of the empirical formula CF 2 in case you forgot what the molar mass of that is it's right here 50 point zero one okay we divide this by this and we get four here's what this four means it means that we take the molar mass of the empirical formula and we have to multiply it by 4 to get the mole mass of the compound that we're solving for we do the same thing to get the molecular formula of the compound we start with the empirical formula CF 2 and we multiply it by 4 to get C 4 it's a 4 because with the C here if there isn't a number after it it means that there's just one of them so it's essentially doing 1 times 4 is 4 and then we do F 2 times 4 is 8 C 4 f 8 is the molecular formula of the compound that has a molar mass of 200 point 0 4 grams per mole it has CF 2 is its empirical formula let's do a couple more examples using just this method without having to do the chart for each one calculate the molecular formula of a compound with the empirical formula C 2 H 2 O and a molecular mass of 126 0.12 grams per mole we'll use this formula to figure out what we have to multiply the empirical formula by to get the molecular formula okay so we'll start with a molar mass of the molecular formula which is given to us in a problem 126 point 1 2 divided by the molar mass of the empirical formula so here's how we'll do it we add 2 times molar mass of carbon plus 2 times the molar mass of hydrogen plus 1 times the molar mass of oxygen and get 42 0.04 we do this division here and we get 3 3 is what we have to multiply the molar mass of the empirical formula by to get the

Loading, please wait...