The relative molecular weight (Мr) is calculated as the sum of the relative atomic masses (Аr) of all the atoms of the chemical elements that make up the molecule. We find the relative atomic masses in the periodic table of chemical elements (the number given in the table is rounded up to an integer).
Mr (Na2CO3) = 2 * Ar (Na) + Ar (C) + 3 * Ar (O) = 2 * 23 + 12 + 3 * 16 = 46 + 12 +48 = 106.
Mr (SO2) = Ar (S) + 2 * Ar (O) = 32 + 2 * 16 = 32 + 32 = 64.
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