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The molecular weight, or the relative molecular mass, is just the sum of the weights of the atoms! Either you can just get the mass of oxygen by itself (15.999) or do it diatomically (15.999*2) Either one your question asks for!
In chemistry, the formula weight is a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a chemical formula by the number of atoms of that element present in the formula, then adding all of these products together. Finding molar mass starts with units of grams per mole (g/mol). When calculating molecular weight of a chemical compound, it tells us how many grams are in one mole of that substance. The formula weight is simply the weight in atomic mass units of all the atoms in a given formula. The atomic weights used on this site come from NIST, the National Institute of Standards and Technology. We use the most common isotopes. This is how to calculate molar mass (average molecular weight), which is based on isotropically weighted averages. This is not the same as molecular mass, which is the mass of a single molecule of well-defined isotopes. For bulk stoichiometric calculations, we are usually determining molar mass, which may also be called standard atomic weight or average atomic mass. A common request on this site is to convert grams to moles. To complete this calculation, you have to know what substance you are trying to convert. The reason is that the molar mass of the substance affects the conversion. This site explains how to find molar mass. Formula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction. These relative weights computed from the chemical equation are sometimes called equation weights. If the formula used in calculating molar mass is the molecular formula, the formula weight computed is the molecular weight. The percentage by weight of any atom or group of atoms in a compound can be computed by dividing the total weight of the atom (or group of atoms) in the formula by the formula weight and multiplying by 100. Using the chemical formula of the compound and the periodic table of elements, we can add up the atomic weights and calculate molecular weight of the substance.
The definition of formula weight is that it is the sum of all of the atomic weights of the atoms in the formula unit (i.e. the chemical formula for the species). It's not really clear what you're trying to ask here...if you are referring to atomic oxygen (i.e. individual O atoms), then we express its weight in atomic mass units (amu) or, more commonly, grams per mole. There is only one atom involved, so we would correctly refer to it as the atomic weight. Each of the oxygen atoms are individually too small and too light to weigh on their own, so we resort to expressing the weight of a sample of oxygen atoms, given that we know exactly how many are present. In this case, grams per mole refers to the weight of a sample containing 6.022 x 10^23 oxygen atoms. We can find this on the periodic table as being approximately 16.00 grams per mole (often written as 16.00 g/mol). If we are talking about the formula weight of oxygen, as opposed to the atomic weight, then oxygen must be in a molecule where it is bonded to other oxygen atoms. There are two cases of this - oxygen gas (which we breathe in), which has a molecular formula (formula unit) of O2, and ozone (in our atmosphere) which has a molecular formula of O3. So, the formula weights of theses two species would be the sum of the atomic weights of the oxygen atoms present in each case. So, given 1 mole (6.022 x 10^23 atoms) of oxygen weighs 16g, 1 mole of ozone (6.022 x 10^23 molecules), for example will weigh (16g + 16g + 16g) = 48g, as each molecule of O3 contains three oxygen atoms. Hope that helps! :)
Thanks for all the help!