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FORMULAS, PERCENT COMPOSITION & HYDRATES FORMULAS Coefficient: A whole number written before a compound. It represents how many molecules you have. 3 H2O means we have ______________________ Subscripts: A whole number written after a chemical symbol. It represents how many atoms of that element are in each molecule. An absent subscript implies only one of the atom is present. H2O means we have ____________________________________ Bringing coefficients and subscripts together: 3 H2O means we have: H2O H2O H2O This equals _______________________________________________ Formula mass = the sum of the ______________ of all of the atoms present (the term molecular mass is often used, but does not really include ionic compounds and network solids, so the term formula mass is preferred) NaBr = 1 Sodium and 1 Bromine atom ___ amu + ___ amu = ____ amu Find the formula mass of each: 1. HCl _____ 2. Mg(NO3)2 _____ 3. LiNO3 _____ 4. KMnO4 _____ 5. Al2(CrO4)3 _____ Gram Atomic Mass The atomic mass of an element expressed in ____ Gram formula mass = formula mass expressed in _________ Na2CO3 = Empirical Formula represents the _________________________________ in which atoms combine to form a compound Molecular Formula represents the actual ratio of atoms in a specific compound. It may be a multiple of the empirical formula. _____________!! To determine the empirical formula from the molecular formula, ________ ___________________________________________________________. Determine the empirical formula for each of the molecular formulas: 1. C8H18 ___________ 2. H2O2 ___________ 3. C3H6O3 ___________ 4. Na2C2O4 ___________ 5. C7H12 ___________ To determine the molecular formula from the empirical formula: * Determine the empirical formula mass * Divide the molecular mass by the empirical formula mass to determine the multiple * Multiply the empirical formula by the multiple Molecular Formula = Empirical Formula x molecular mass empirical mass Example: The molecular mass of propene is 42amu. The empirical formula is CH2. What is the molecular formula? Determine the empirical formula mass The empirical formula mass is ________. Divide the molecular mass by the empirical formula mass to determine the multiple Molecular form mass (____) ÷ empirical form mass (____) = multiple of ___ Multiply the empirical formula by the multiple Multiplying the empirical formula x 3 = _________ 1. A compound with an empirical formula of CH2O has a molecular mass of 90 amu. What is its molecular formula? 2. Find the molecular formula for a compound with a mass of 78 amu and the empirical formula CH. 3. Find the molecular formula for a compound with a mass of 82 amu and the empirical formula C3H5. 4. Find the molecular formula for a compound with a mass of 90 amu and the empirical formula HCO2. 5. Find the molecular formula for a compound with a mass of 112 amu and the empirical formula CH2. 6. Find the molecular formula for a compound with a mass of 40 amu and the empirical formula C3H4. PERCENT COMPOSITION Percent composition = the percentage of each element in a compound Mass of one element (part) Mass of the compound (whole) X 100% = percent composition Determine the percent composition of each element in each of the following compounds: 1. Pb(OH)4 Pb ______ O _______ H _______ Total Mass _______ 2. Fe(C2H3O2)2 Fe _______ C _______ H _______ O _______ Total Mass _______ 3. NaOH Na______ O______ H______ Total Mass _______ 4. Which of the following has the highest percentage of nitrogen? Ca(NO3)2 (NH4)2SO4 Hydrates = crystals that contain a definite number of _________________. In a chemical reaction, the water does not react. However, it adds mass to the compound. CuSO4 · 5H2O copper sulfate pentahydrate Anhydrate = Treat the water molecule as one!! What is the percentage, by mass, of water in copper sulfate pentahydrate? Formula mass of CuSO4 Cu = 64 x 1 = S = 32 x 1 = O = 16 x 4 = Total = Formula Mass of H2O H=1x2= O = 16 x 1 = Total of each water molecule = Total of 5 water molecules = Total CuSO4 + 5 H2O = Mass of water______ X 100 = Mass of entire compound 1. What is the percentage of water in CaCl2 · 10 H2O? 2. What is the percentage of water in CaSO4· 6 H2O?
