Writing a chemical recipe

When we make a cake, the recipe tells us, for example, that we will need 100 g of sugar for every 100 g of butter. It is the same for a chemical reaction. When we write a balanced chemical equation, e.g. A + B — C + D, we are really saying that one particle of A reacts with one particle of B to form one particle each of C and D. Chemical reactions occur in larger quantities than single atoms, so the equation also says that one mole of A reacts with one mole of B, etc., because we know that one mole of anything contains the same number of particles as any other mole. Statements about atom ratios in equations or formulae are statements about mole ratios. We can be sure that there are the same number of particles present if we weigh out the correct mole proportions as set out in the balanced equation. The idea of the mole allows us to count particles simply by weighing. 

This tells us that 2 molecules of hydrogen chloride (in hydrochloric acid) will react with 1 formula unit of calcium carbonate (as it contains a metal it must be an ionic compound), but it does not tell us how much hydrogen chloride in grammes we must add to the calcium carbonate to ensure that all the calcium carbonate is used up. But if we use moles all is much clearer. The relative formula mass of calcium carbonate is 40 (from calcium) -1-12 (from carbon) -I- (3 X 16) (from oxygen) = 100. So 1 mole of calcium carbonate has a mass of 100 g. Then for every 100 g of calcium carbonate we are reacting we will need to use 2 x 36.5g(=73g) of hydrogen chloride. 

Let us look again at the balanced reaction equation above with some actual masses. (From the periodic table, Ca has a RAM of 40 Cl, 35.5 C, 12 O, 16, and H, 1.) If we use these quantities, then because the masses relate to the number of particles, we will have the correct number of each of the different particles to satisfy fully all of the reacting materials. 

Of course, we don t very often react such large quantities as a whole mole in the laboratory, we have to scale it down, just as you may have to do in following a recipe in the kitchen. If the recipe is for four people and there are only two of you, you divide all the quantities of ingredients by two. The same is true of chemical recipes. Let us look at the last example and scale it down to /loth scale. 

The unit for the mole is mol. In future examples and sections this unit will be used in equations. 

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