The Law of conservation of mass states that matter cannot be created or destroyed in any chemical reaction The atoms in the reactants are rearranged to form new compounds, but none of the atoms disappear, and no new atoms are formed. H H H H O O Reactants Products

A chemical equation identifies the starting and ending chemicals as reactants and products: reactants  products Formation of water 2H 2 + O 2  2H 2 0 A chemical equation is balanced when it reflects the conservation of mass. Graphic: state.edu/betha/nealChemBal/

The same number of each kind of atom must be on the left side of the arrow as are on the right side when an equation is balanced. Al OOOOOO OOO OOO

Oxygen, as a product, must be multiplied by 2. A coefficient is placed in front of the compounds that contains O. Oxygen atoms are = When a coefficient is added you must multiply all atoms in the compound by this number. Redo the inventory After redoing the inventory, add coefficients to balanced other atoms. With each additional coefficient the inventory must be updated. Oxygen atoms are ≠ Hydogen atoms are = 2 H atoms 2 H atoms 2 O atoms 1 O atom Hydogen atoms are ≠ Oxygen atoms are = First: Inventory the atoms on both sides of the yield arrow Balancing an equation: H 2 + O 2  H 2 O H H H O O O H Second: Determine which atoms are not equal and add coefficients to balance these atoms. 2 4 H atoms 2 O atoms 2 4 H atoms 2 O atoms Hydogen atoms are = This is now a balanced equation

So: Chemical equations are balanced, when the numbers and kinds of atoms on each side of the reaction arrow are equal. H H H H O O Reactants Products H H H H O O 4 Hydrogen 2 Oxygen

Because atoms are conserved, the mass of the reactants in the chemical reaction is equal to the mass of the products. H 2 + F 2  2 HF 2g + 38g = 2( 20g) 40g = 40g 40 g Reactants Products HH FF H F H F

The Law of Conservation of Mass allows the calculation of the reactants or products. If 27.0 g of mercuric oxide (the red solid compound) is heated and completely decomposes to give the elements oxygen and mercury. 2.0 g of oxygen are produced. How many grams of mercury are produced? mercuric oxide  mercury + oxygen HgO  Hg + O 2

Reactants Products 27.0 g HgO ? g Hg 2.0 g O 2 27 g HgO = 2 g O 2 + ? Hg = 25 g Hg

Which of the following chemical reactions indicate that mass in conserved? A.Mg + Cl 2  2 MgCl B.2 Ca + O 2  2 CaO C.Zn + S  2 ZnS D.C + O 2  2 CO The correct answer is B Ca O O O O Because atoms must be equal on each side of the equation:

Mg + HCl → MgCl 2 + H 2 When the above equation is balanced, the coefficient for magnesium is — A 0 B 1 C 2 D 4 2 Mg + 2 HCl → MgCl 2 + H 2 The correct answer is C

Suppose 100 g of iron metal rusts. We weigh the rust and find that the rust has a mass of 143 g. What mass of oxygen reacted with the iron? A. 243 g B. 57 g C. 100 g D 43 g 43 g Oxygen Choice D Iron + Oxygen  Rust 100 g + ?g  143g What mass of oxygen is needed?