Law of conservation of mass: matter is neither created
nor destroyed during a physical or chemical change
or….What goes in must come out.
Ex 1: 23 g of sodium metal is reacted with 35 g of chlorine gas to form sodium
chloride. How much sodium chloride was formed?
reactants
products
23 g sodium + 35 g chlorine
58 g of sodium chloride
Ex 2: 56 g of iron reacts with oxygen to form 80 g of iron oxide. How many
grams of oxygen were used in the reaction?
56 g iron + x g of oxygen
80 g of iron oxide
x = 24 g of oxygen
Mass% =
mass of the part
x 100
mass of the whole
Ex 1: A sample of water contains 2 g of hydrogen and 16 g of oxygen. What is
the mass% of hydrogen and oxygen in water?
2g
%H 
x 100  11% H
2 g  16 g
16 g
%O 
x 100  89% O
2 g  16 g
Ex 2: A compound containing only carbon and oxygen is 27% carbon. How
many grams of oxygen are present in a 100 g sample of the compound?
 If 27% of the compound is carbon, then 73% of the compound is
oxygen.
 73% of the 100 g sample weight  73 g of oxygen
Hydrates: a crystalline material that contains a fixed percentage of
water molecules within its crystal structure.
An anhydrous crystal contains no water
Solids can have a variable amount of water adsorbed
onto their surfaces.
A crystal hydrate has a constant amount of water
molecules within its crystal lattice.
Ex 1: 243 g of barium chloride dihydrate (BaCl2•2 H2O) are
heated until the compound is anhydrous. What is the final
weight of the compound if the hydrate is 15% water?
15% of compound is water
85% of compound is anhydrous crystal  0.85 (243 g) = 207 g
Ex 2: 141 g of a hydrate are heated strongly until a constant
weight of 60 g is obtained. What was the %water in the
hydrate?
Mass of water = 141 g hydrate – 60 g anhydrous = 81 g of water
%H2O 
81 g H2O
141 g hydrate
x 100  57% H2O