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Molecular Weight

In chemistry, a molecule is a discrete unit consisting of two or more atoms covalently bonded to each other. Molecules are distinguished from ions by their electrical charge, mass, and other properties. The smallest molecule is the hydrogen molecule, H2, which consists of two hydrogen atoms covalently bonded to each other. A molecule may be homonuclear, that is, a single molecule consisting of atoms of one element, or it may be heteronuclear, consisting of more than one element.


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Video Transcript

so molecular weight is calculated based on number of atoms in the molecule. So Molecular Week is actually in a M. U, which is atomic mass unit and is purely a mass quantity where one proton and one neutron is about one amy. So when you're trying to bear out the molecular weight, you need to first figure out the molecular formula. And then you will look at the periodic table to figure out the Amy's for each item and then, based on your molecular formula, calculate the molecular weight and as a note, whenever you're again some kind of value, you need to note the number of sick pigs so your answer should never include for sig figs and what you are given, because you are adding degrees of certainty that are not certain for the values that you are using. So, for example, if you are given three quantities with different numbers of sig figs, your answer must use three lower number of sick pigs out of the three that you argument eso going to our first example. Let's say you want to figure out the atomic Sorry, the molecular weight of H 20 eso. You know that hydrogen has an atomic mass unit of 1.8 and oxygen has an atomic mass unit of 6.0 So in this case, you're given two quantities with forcing figs. So you want to make sure that your final answer and a muse is forcing fix. Um, So according to our molecular formula, we know that there are two Hodgins Adams. So that means we need, um to include that in our molecular weight so we can write. This has two times 1.8 added to 16.0 AM use. So then your final answer would be 18.2 AM use. So this is the molecular weight of water. Um So for another example, let's say you want to figure out the molecular weight of you want to figure out the molecular weight of sudden import, so using the same method, let's go back to our periodic table to figure out the A muse for each atom in our molecule. So in this case, Syrian has a atomic mass unit of 22.99 and chloride has an atomic mass unit of 35 0.45. So again, to calculate the molecular weight you want, Thio add up all of the atoms that make up your molecule on to obtain it. So here we just have one sodium Adam so we can write 22.99 and then add that to another single chloride, Adam, which is 35 0.45 to get a final answer of 58 0.45 AM use. So again, we use the same method to figure out the molecular weight a k a. Figuring out the molecular structure as well as the in use for all of its components and then accounting for the different number of atoms for each element used to obtain the molecular weight. And for our last example, let's say we want to figure out the molecular weight for sure. So, like the examples before sugar is C 12 h, 22 11, and so we need to figure out the a muse for carbon, hydrogen and oxygen. Eso Harmon has an atomic mass, has atomic mass units of 12.1. Hydrogen is 1.8 on. Let's make sure that we have the right units just to remind ourselves again that this is purely a mass quantity. Okay, so for oxygen, it's 6.16 point 00 and use. All right, great. So now we have all of the muse Were all the items that were that we need for this problem and so we can go on to figure out the molecular weight. So there are 12 carbon atoms, and so we need to multiply 12 by the atomic mass unit off a single carbon. And then we also need to do the same thing for hydrogen. So there are 22 hydrogen and sugar. So we need to make sure that we multiply this by the atomic mass unit of a single hydrogen. And lastly, we need to account for the number of oxygen's. And there are 11 oxygen atoms. So if we multiply this by the atomic mass units early, single oxygen and add all of these together, we should be able to get the molecular weight of sugar, which turns out to be 342 point to a muse. And again, um, like I said before, we want to make sure that our answer is in the lowest number of sig figs. Um, depending on what you are given. So again, you're given your am use with four sig figs. So then your answer should also have 466