Percent Composition and Molecular Formulas

 Determining the percent composition of each element in a compound  H 2 O 1. Find the molar mass of the compound 2. Find the mass of each element in the compound (remember the subscript) 3. To find the % composition by element you must divide the mass of the element by the molar mass of the compound. Multiply by 100

Mass of elementX 100 Mass of Compound

 The simplest whole number ratio of the combination of elements  The empirical formula for hydrogen peroxide is HO  The molecular formula for hydrogen peroxide is H 2 O 2  The percent composition can be used to calculate the empirical formula. Assume that the mass of the compound is 100g if not indicated

 Assume the percent composition is the mass in grams if the total % for the compound = 100  Example: SO 3  %S = = 40.05g%O = = 59.95g 1. Find the # of moles of each element in the compound. Do not round the # of moles to a whole number 2. Divide the smallest mole amount by itself to = 1 3. Divide the remaining moles for other elements by the smallest mole amount 4. If a whole number still needed multiply all mole amounts by the smallest factor needed to get a whole number for all elements

48.64% carbon, 8.16% hydrogen and 43.20% oxygen

 36.84% N, 63.16% O

 60.00% C 4.44% H 35.56% O

 g Ni 4.38g C 5.10g N

 The molecular formula specifies the actual number of atoms of each element in one molecule or formula unit of the substance  To determine the molecular formula  1. Find the n (n=# used to multiply empirical formula by) Actual molar mass of the compound = n Empirical Molar Mass of Compound 2. Multiply by the n Molecular Formula = (empirical formula)n

 40.68% C 5.08% H 54.24% O  Compound has a molar mass = 118.1g/mol

 65.45% C 5.45% H 29.09% O  Molar mass = 110.0g/mol

 Hydrate – compound that has a specific number of water molecules bound to it  Naming a Hydrate – use a prefix to indicate the number of the hyrdate  Molar Mass of a Hydrate – is the molar mass of the compound plus the water mass  CaCl 2 · 2H 2 O  Calcium Chloride dihydrate  ℳ = g

1. Find the number of moles of the compound and the water from the mass given 2. Find x (the moles of water) x = moles H 2 O moles of compound (anhydrous) 3. Use x as the coefficient for moles in the hydrate formula Ex: if x = 2 then 2H 2 O dihydrate

 2.5g blue hydrated CuSO 4 ·xH 2 O is heated. After heating, 1.59 g white anhydrous CUSO 4 remains. What is the formula for the hydrate? Name the hydrate

 48.8% MgSO % H 2 O