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The Law of Conservation of Mass

The Law of Conservation of Mass. Recall. One part of the Atomic theory states that matter can not be created or destroyed by ordinary means. We have just investigated Ionic bonding a form of chemical reaction. We are now going to investigate other chemical reactions.

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The Law of Conservation of Mass

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  1. The Law of Conservation of Mass

  2. Recall • One part of the Atomic theory states that matter can not be created or destroyed by ordinary means. • We have just investigated Ionic bonding a form of chemical reaction. • We are now going to investigate other chemical reactions. • Note: ALL chemical reactions are Ordinary means. • See Overheads.

  3. The Law of Conservation of Mass • Since Chemical reactions are ordinary means, then the matter in a chemical reaction can not be created or destroyed. • This means that the matter you start with has to still exist after the reaction. • The reaction must balance.

  4. The Chemical Equation • We represent a reaction by writing an equation. • Ex. • CdS + HCl => CdCl2 +H2S • This equation shows that Cadmium Sulfide reacts with Hydrogen Chloride (Hydrochloric Acid) to form Cadmium Chloride and Hydrogen Sulfide. • REACTANTS: The initial chemicals that go through the chemical reaction. ( In the above example; CdS + HCl ) • PRODUCTS: The chemicals that are left after a chemical reaction. ( In the above example; CdCl2 +H2S)

  5. The Law and the Equation • CdS + HCl => CdCl2 +H2S • Note every element that exists as a reactant also exists as a product. • Cd on left and Cd on right • S on left and S on right • H on left and H on right • Cl on left and Cl on right. • Note: No element disappears and no new element if formed.

  6. Balance • If all of the reactants still exist as products. • Then the exact same atoms that are there to start with must still be there when done. • Since atoms are matter and matter is mass then we can make this statement. Total Mass of Reactants = Total Mass of Products.

  7. The Law and Quantitative Measurements. • Given: • CdS + HCl => CdCl2 + H2S • 6.0 g 7.0 g 8.0g 5.0g • Do these equal? Lets total both sides. • Total Reactants = 6.0 g + 7.0 g = 13.0 g • Total Products = 8.0g + 5.0 g = 13.0g • 13.0g = 13.0 g • Total Reactants = Total Products

  8. Usefull • This technique is very useful if you need to know how much reactants to use to make a certain amount of product, or, how much products to expect when you have a given amount of reactants. • CdS + HCl => CdCl2 + H2S • 6.0 g 7.0 g ? 3.0g • Rewrite this as a mathematical equation. • 6.0 g + 7.0 g = x + 3.0g • 13.0 g = x + 3.0g • 10.0 g = x the mass of the CdCl2

  9. You Try • Complete the Law of Conservation of Mass practice sheet #’s 1,2, 4 (3 for bonus). • You will need to remember %. • % = Part x 100 Whole • If there are 4.0 g of Cd in CdS and there are 6.0 g total, what % by mass is there of Cd in CdS? • % = 4.0 x100 = 0.667 x 100 = 66.7 % 6.0

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