mass percent composition is another way that we talk about the makeup of a molecule and the formula for calculating this mass percent of element x is what i've put here for my generic so the mass percent of element x in a compound would be equal to the mass of that element divided by the mass of the compound and then multiply by 100 to give a percentage so this is best demonstrated with an example and our example is what is the mass percent of sodium in sodium chloride one molecule of sodium chloride would have one sodium that weighs 22.99 grams per mole and one chlorine that weighs 35.45 grams per mole so the entire molecule would weigh 58.44 grams per mole out of that so if i you kind of pretend that you have one mole so it would be 58.44 grams however we can just kind of put these values in with the grams per mole and if you're comparing them you would assume that it's the same number of moles for both and then our units are going to cancel out anyway so 58.44 grams per mole is the total that goes in the denominator the sodium content of that molecule it's one sodium at 22.99 the grams per mole cancel this would give me 0.3934 and multiplying by 100 gives me 39.34 sodium by mass so why would this even matter what would we do with this now you'll see this come up in a bunch of different places including in lab if you're taking lab this percent by mass it's an easy way to make some solutions it's an easy way to figure out how much of something is in a big sample like maybe you want to extract it or like the example we're going to do maybe you're not supposed to eat that much of it so the example we're going to look at has to do with fda recommendations and they recommend consuming less than 2 300 milligrams of sodium per day and i'm fairly certain at this point that this is actually an old recommendation i think the new recommendation is lower than this so the question here is if a person consumes eight grams of sodium chloride how much sodium did they consume and then let's see how it compares to the fda recommendation one of the seemingly endless ways in which you could set up this calculation is looking at the mass percent of the mass given so if you have a sample of 8 grams of sodium chloride if it's pure sodium chloride 39.34 of that sample is sodium so 0.3934 times 8 gives you 3.1472 grams of sodium that's which is a lot but it was an 8 gram sample so that kind of makes sense now the recommendation is given in milligrams so you have two options here you could convert your grams of sodium to milligrams and compare or you could convert the recommendation to grams and then compare those i've converted the sodium to milligrams but again either would be fine so 3.1472 grams of sodium there are 1 000 milligrams in one gram so after this conversion that's 3147 milligrams of sodium that's way too much that's where we're approaching we're at least at like 1.5 times the daily recommendation so this is an example of how you would use a how you calculate a mass percent and then also use it to do a conversion or calculation