# Calculation Chemistry

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• Calculation Chemistry
• Ar (Relative Atomic mass) based on 1/2th of a Carbon-12 isotope. Basically just the mass number on periodic table
• Mr (relative molecular mass.) The mass of a whole molecule. Add mass of all the different elements in the molecule.
• Weighted Average: Multiply the relative abundance (the percentage of that isotope compared to the rest of that element) by the Relative Atomic Mass. Do for each isotope and add the results. Divide this answer by 100.
• Moles: Unit for amount in chemistry, takes into account that atoms are so small.
• Avogadros Number: 6.02x10 to the power of 23. The amount of particles in a mole.
• 1 mole of any monatomic element weighs the same as its Ar in grams (e.g. Fe). 1 mole of any compound or diatomic element (e.g. O2 or CO2) weighs the same as its Mr in grams.
• EMPIRICAL FORMULAE: The simplest whole number ratio of elements in a compound. E.g. 2.7g of Al is combined with 10.65 of chlorine. What's the empirical formula? Step1: Divide masses by Ar so 2.7/27 and 10.65/35.5. This gives number of moles. Then divide by smaller answer so 0.1/0.1 and 0.3/0.1. This gives the ratio, in this case 1:3. So the empirical formula is AlCl3.
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