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# Moles, Atom Economy and their Calculation

[highlight][b]Mole[/b] - Chemists measure the amount of a substance in a unit called 'the mole'. It is a way of counting atoms and also allows chemists to make predictions about the masses of different substances in reactions. The relative atomic mass of carbon is 12, meaning that in one mole, you would have 12g of carbon.[/highlight] [code]Mole = Mass [divide] Mr[/code] [i](Mr is the relative atomic mass of an element and is the greater number next to the element on the periodic table.)[/i] For example: [i]How many moles of chlorine are there if you have 71g of it gained in a reaction?[/i] Since the Mr is 35.5, you would do: [code]71 [divide] 35.5 = [b]2 moles[/b][/code] [highlight][b]Avogadro's number[/b] - One mole is the 'Avogadro's number of particles in a substance. One mole of atoms contains 6X10 to the power of 23 atoms, no matter what.[/highlight] [highlight]Yield - The amount of a substance you obtain at the end of a chemical reaction[/highlight] [highlight]Percentage Yield - The percentage of actual yield over theoretical yield. [code]Actual [divide] Theoretical X 100[/code][/highlight] E.g: 46 grams of Sodium(Na) reacts with water(H2O) to produce Sodium Hydroxide(NaOH) and Hydrogen(H2). You need to find the percentage yield for sodium. Here is the balanced symbol equation: Na + H20 ---> NaOH + H2 If the theoretical yield for sodium Hydroxide is 160g, work out the percentage yield for Sodium. Step 1) Find the number of moles of sodium (which is = to the moles of sodium hydroxide because of the 1:1 ratio) = Mass/Mr = 46/23 = 2 moles Step 2) Find the Mr for sodium hydroxide (product) = 23 + 16 + 1 = 40 Step 3) Find the actual yield (undefined formula) = Moles X Mr = 40 X 2 = 80 grams Step 4) Find percentage yield = Actual mass/theoretical mass X 100 = 80/160 X 100 = 50% Atom economy - the conversion efficiency of a chemical process in terms of all atoms involved and the desired product produced. It is the percentage of a wanted substance in the reactants or products. Atom economy = wanted compound or atom/reactants or products X 100 E.g; Na + H20 ---> NaOH + H2 If there is 46 grams of sodium at the beginning of the reaction, and the mass of all the products at the end of the reaction adds up to 100 grams, what is the atom economy for sodium hydroxide at the end of the reaction. Step 1) Find the moles of sodium (which is equal to the moles of sodium hydroxide because of the 1:1 ratio)= Mass/Mr = 46/23 = 2 moles Step 2) Find the mass of Sodium Hydroxide = Moles X Mr = 2 X (23 + 16 + 1) = 80 grams Step 3) Find the atom economy = Wanted compound/Products X 100 = 80/100 X 100 = 80%