So the final answer should be reported as 180.16 g/mol. So the 5 digit get upgraded to a 6 in the answer. Since the number we're discarding (6) is larger than 4 we round the digit it is next to up one number. So we only want the 1 and 5 digits and want to discard the 6. But because we only have two decimals digits for the 72.06 and 96.00 numbers, our answer is limited to two decimal digits. So after multiplying Sal is performing this calculation: 72.06 + 84.156 + 96.00, which mathematically would yield 180.156. For addition the sig fig rule is that we can only have as many decimal digits as do in the calculation number with the lowest number of decimal digits. You should look into sig figs in greater detail, but in this problem Sal is adding three numbers together. And the rules we use to judge how many digits are permissible are significant figures, or sig figs. But in science we are more conservative with the digits we use because of the precision of our measuring instruments. After a calculation would be able to use all the digits of the final number as our answer if we only wanted a mathematical answer. The formula weight is simply the weight in atomic mass units of all the atoms in a given formula.It's important to keep in mind significant figures are important for doing calculations in a science like chemistry. When calculating molecular weight of a chemical compound, it tells us how many grams are in one mole of that substance. This site explains how to find molar mass.įinding molar mass starts with units of grams per mole (g/mol). ![]() The reason is that the molar mass of the substance affects the conversion. To complete this calculation, you have to know what substance you are trying to convert. 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.Ī common request on this site is to convert grams to moles. For bulk stoichiometric calculations, we are usually determining molar mass, which may also be called standard atomic weight or average atomic mass. This is not the same as molecular mass, which is the mass of a single molecule of well-defined isotopes. ![]() This is how to calculate molar mass (average molecular weight), which is based on isotropically weighted averages. ![]() The atomic weights used on this site come from NIST, the National Institute of Standards and Technology. 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. ![]() If the formula used in calculating molar mass is the molecular formula, the formula weight computed is the molecular weight. These relative weights computed from the chemical equation are sometimes called equation weights. 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.įormula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction.
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