Here is where we will begin our journey to periodicity. One such discovery was that of atomic masses. The modern periodic table didn’t spring fully formed from the genius of Mendeleev it was shaped by key discoveries about the elements. So how did Russian chemist Dmitri Ivanovich Mendeleev and the other creators of the periodic table (arguably six of them) bring order to the elements? How did they create a tool that would ultimately house 118 elements when they knew only 62 of them? And why does Mendeleev get all the credit? Mass + reactions = periodic table It even predates knowledge of the noble gases. However, the periodic table predates knowledge of atomic numbers and subatomic particles (yes, including electrons). If you know about atomic numbers and electron shells, recreating the periodic table is simple. You would order them by increasing atomic number and create a new row when you hit a noble gas. If I gave you all of the elements on cards and told you to recreate the periodic table, you probably wouldn’t have much trouble. You can compare electronegativity, ionization energy, atomic radius, chemical reactivity, and more. You can determine the electron configuration of any atom, simply from its place. M = 58.7 + 2 x 35.5 + 6 x 18.0 = 237.7 g/molĬalculating the molar mass is essential as it lays the foundation for most of the calculations in stoichiometry such as the mass, moles, number of particles and etc.The periodic table is an elegant demonstration of properties of elements. He formula of the hydrate is NiCl 2♶H 2O, and therefore, the molar mass would be: To calculate the molar mass of a hydrate, determine the molar mass of the salt and add to the molar mass of the water multiplied by its subscript.įor example, calculate the molar mass of nickel (II) chloride hexahydrate. The formula of nitrate ion is NO 3 –, therefore, the molar mass would be: It is a result of either losing or gaining an electron(s) which has negligible mass compared to atoms and molecules.įor example, what is the molar mass of the nitrate ion? Remember, the number outside of parenthesis applies to all the elements in it. Next, we write the ions next to each other and add the correct subscripts:Īnd now, we multiply the molar mass of each element by its subscript. You need to know the common polyatomic ions from where we find that sulfate is the SO 4 2- ion. M (PCl 5) = 30.1 + 5 x 35.5 = 207.6 g/molĪnother example: what is the molar mass of aluminum sulfate?įirst, we know that the aluminum ion has a 3+ charge. To calculate the molar mass of PCl 5, we multiply the molar mass of each element by its subscript: Go over naming covalent compounds to refresh some of the concepts here. The formula of phosphorous pentachloride is PCl5. If the formula of the molecule is not given, you will need to first determine it.įor example, determine the molar mass of phosphorous pentachloride. To calculate the molar mass of a molecule, we add the molar mass of each constituent atom by the corresponding subscript.įor example, the molar mass of water would be: Usually, we round off the molar mass to 1 or 2 decimal places, so for P, we’d use 123.9 or 123.90 for its molar mass. For example, to find the molar of P 4, we multiply 30.974 g/mol by 4: If there is a subscript in the symbol, then you need to multiply the molar mass by that number. And we can do this for all the elements in the periodic table. For potassium then, it is 39.098 g/mol which means one mole of potassium atoms weighs 39.098 g. What’s important for today’s topic is that numerically, the molar mass is equal to the average atomic mass of the given atom, so we can look it up in the periodic table. The smaller one is the atomic number which shows the number of protons, and the larger one is the average atomic mass: So, how do we find the molar mass in the periodic table?įor example, there are two numbers given for potassium in the periodic table: 19 and 39.098. To calculate the molar mass of an atom or a molecule, we are always going to need a periodic table. The mass of one mole of atoms/molecules/ions is called its molar mass ( M) expressed in g/mol.
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