Elemental analysis is just one of many different types of analytical techniques that chemists can use to determine the composition of the sample. Elemental analysis is quantitative. In other words, we can use it to determine how much of each type of atom there is in a compound. With this technique, chemists can determine mass fractions, or mass percents. This is useful information if we're trying to identify an unknown substance, or if we're trying to determine the purity of a known compound. For example, let's look at a common household chemical, hydrogen peroxide. Now normally, hydrogen peroxide is diluted to between three and 6% in water. And it's delivered, in these opaque containers because it's light sensitive. It cleaves in the presence of light. When I was a child, my mom used to pour hydrogen peroxide on my rooms as a disinfection process and it really hurts. If you find that you pour hydrogen peroxide on your room and it doesn't hurt, that's probably because the sample is really old and most of the hydrogen peroxide has decomposed to water. Hydrogen peroxide has the chemical formula H2O2. We could determine what the percent by mass of hydrogen is, and what the percent by mass of oxygen is. That would be a calculation of mass percent. But before we do that, we need to know the mass of the entire molecule. This is a review of something you've already learned how to do in the course. So why don't you try to go ahead and do that now? What is the molar mass of hydrogen peroxide? Very good. Most of you were able to calculate the molar mass. Which is also sometimes called the formula mass or the molecular weight. The following way. There's two hydrogen atoms. So we say 2 times 1.0079 grams per mole. There's also two oxygen atoms, and the mass of an oxygen atom is 15.999 grams per mole. We add those together, and we got 34.0138 grams per mole of hydrogen peroxide. Now, to determine the mass percent of hydrogen and the mass percent of oxygen, we need the following formula. The percent by mass of a given atom is the mass of that atom divided by the total mass times 100%. So for hydrogen peroxide, the mass percent of hydrogen is the mass of hydrogen in the compound divided by the total mass of the compound. Now we know that the mass of one hydrogen is about one gram per mole, but hydrogen peroxide has two atoms of hydrogen. So the mass of hydrogen in hydrogen peroxide is 2.0158 grams per mole, because there's two hydrogens. We have to divide that by the molar mass of the entire compound. Multiply that by 100%, and we see that the mass percent of hydrogen in hydrogen peroxide is 5.926%. The calculation for oxygen is really similar. Again, there's two oxygens in the compound. So, there would be 31.998 grams per mole of oxygen in hydrogen peroxide. We divide that by the total mass of the compound, multiply it by 100%, and get the mass percent of oxygen in hydrogen peroxide being about 94%. We can check our work by adding the mass percent of all the elements up. And they should equal 100%. So if we add up 5.926 plus 94.074, we see that that does in fact equal 100%, so we did the calculation properly. Now it's your turn to try one. Methane is a flammable gas, it's a component of natural gas. If we had a ten gram sample of methane, how many grams of carbon would be in this sample? Methane is actually in the news quite a bit, because as a component of natural gas, we do different things to try to mine natural gas. One of the things that's in the news in my local area at the moment is a technique for mining called fracking, so you might want to look up what that is. It's also called hydraulic fracturing. And look at how they use that to get methane from the ground. In some parts of the world, methane just comes up to the surface of the earth through natural vents. But in other parts of the world, methane is trapped, for example, in oil shale that's far down under the surface of the earth, and they need to use hydraulic fracturing to get that methane out of those mines. Alright, go ahead and try, a ten gram sample of methane, how many grams of carbon are in that sample? The first thing we need to do, to do that calculation, is determine the molar mass of methane. There's one carbon atom and four hydrogen atoms. So we've determined the molar mass is about 16 grams per mole for methane. We then used the formula from the previous slide to determine the percent of carbon and the percent of hydrogen by mass. We see that methane is about 74% carbon. And it's about 25% hydrogen. Remember, the mass of hydrogen that we used there was for four hydrogens, not for one. Because there's four hydrogens over here in the formula for methane. If we take the percent by mass of carbon in the methane and multiply that times the amount of methane that's present, ten grams, we see that that sample has 7.49 grams of carbon. It also happens to have 2.51 grams of hydrogen, but that's not what we were asked to calculate. So this would be the answer to the question. Remember we can do the quick check. We can make sure the amounts we ended up with add up to the total amount, and they do. So, that makes me feel good. Here's another practice problem for you to try with mass percent. So, to prevent tooth decay, fluoride is often added to water in the United States. In some parts of the world, the amount of fluoride in the water is already very high. In some cases dangerously high. But in the United States, they started adding fluoride to municipal water sources to prevent tooth decay. It's also present in toothpaste. What is the mass percent composition of fluoride in sodium fluoride? Go ahead and try that now. Fluoride is one of the many cases of the dose makes the poison for chemicals. It can be either beneficial or detrimental to human health, depending on the dose. I found this interesting bumper sticker that someone took a photo of, it says fluoride is a poison, don't put it in my water, but in fact the addition of fluoride to municipal drinking sources has been hailed as one of the top ten health advances of the twentieth century. You can read more about the health effects of fluoride in drinking water in this World Health Organization report, if you're interested. It talks about which countries need to add fluoride to the water, and which countries already have a large amount of fluoride in the water. In some cases, the amount in the water is large enough that it actually needs to be removed. Through a process called defluoridation. But if you don't have enough fluoride in your diet or in your water, one of the things that can happen is tooth decay. And in small doses, the fluouride prevents tooth decay. I don't know how well you can see on this picture, but this poor camel has terrible tooth decay on his bottom teeth. The lethal dose of fluorine for most adults is five to ten grams, which is a very large amount. That amount would cause acute symptoms such as upset stomach or, over a long period of time, would cause chronic problems with bone development. People sometimes do get poisoned by excessive fluoride in water. This can either be fluoride that's leaching out of rock in a well water source or it can be fluoride that was accidentally added in a quantity that was too great to a municipal water source. This has happened a couple times in the United States. And in, in those cases people got very sick. So, this is something to think about. There are chemicals like fluoride, that in small amounts, are extremely beneficial. And in large amounts can be dangerous. So it's important for us, as educated individuals, to understand the difference between a beneficial amount and a dangerous amount. To summarize, various calculations involving mass percent were covered in this lecture. We also reviewed a previous skill that we've learned, the calculation of molar mass for compounds. These types of calculations and converting from grams to moles are routinely used in biology and chemistry labs and also in industrial, environmental and pharmaceutical applications.
