Section 12.2: Using Moles (part 3)

Mass Percent Steps: 1) Calculate mass of each element 2) Calculate total mass 3) Divide mass of element/ mass of compound

Calcualte the mass % of C and H in C 2 H 6 2 mol C x 12 g C = 24 g C 6 mol H x 1 g H = 6 g H Total mass = C 2 H 6 = 30 g % C = 24 g C x 100 = 80% 30 g C 2 H 6 % H = 6 g H x 100 = 20% 30 g C 2 H 6 * Should add up to 100

Example 2 Calculate the mass % of C, H, Br in C 6 H 5 Br 6 mol C x = mol H x = mol Br x = = g % C = g C x 100 = 45.9% g C 2 H 5 Br % H = g H x 100 = 3.2% g C 2 H 5 Br % Br = g Br x 100 = 50.9% g C 2 H 5 B

Chemical Formulas Empirical Formulas Molecular Formulas Empirical: NaCl Molecular: K 2 C 2 O 4 Empirical: KCO2 Molecular: C 20 H 20 O 4 Empirical: C 5 H 5 O

Empirical formula Formula that shows the smallest whole- number ratio of atoms for the compound Steps: 1) Switch % into grams 2) Convert grams to moles for each element 3) Divide by the smallest mole

EMPIRICAL FORMULA A compound is ~26 % N and 74 % O. What is the empirical formula? 26 g N x 1 mol N = 1.86 mol N / 1.86 = g N 74 g O x 1 mol O = 4.63 mol O / 1.86 = g O Can’t have decimal #s (N 1 O 2.5 ) 2 = N 2 O 5

What is the empirical formula of a compound that is 80% C and 20% H of 100 g compound? 80 g C x 1 mol C = 6.67 mol C 12 g C 20 g H x 1 mol H = 20 mol H 1 g H Divide both by smallest: 6.67 mol C = 1.0 mol C mol H = 3.0 mol H 6.67 The mole ratio for this compound is 1 mol C: 3 mol H Formula: CH 3 (Empirical formula)

Molecular formula To determine the molecular formula you need the mass of the compound Steps: 1) First find the empirical formula 2) Divide the total mass by the empirical formula mass 3) Multiply the empirical formula by this #

Example continued: Molar mass is 30 g/mol → molecular mass is 30u Molecular mass of CH 3 = 15 u By dividing the molecular mass you can find the multiple: 30 u = 2 15 u Molecular formula is C 2 H 6

An oxide of Cr is 68.4% Cr by mass. The molar mass is 152 g/mol. What is the formula? = 31.6 g O 68.4 g Cr x 1 mol Cr = 1.32/1.32 = g Cr 31.6 g O x 1 mol O = 1.98/1.32 = g O Cr 1 O 1.5 Cr= O= = / = 2 (Cr 1 O 1.5 ) 2 = Cr 2 O 3

What is the molecular formula for C 4 H 4 O with a mass = 136 g/mol? C: 4 x = H: 4 x = O: 1 x = = /68.07 = 2 (C 4 H 4 O)2 = C 8 H 8 O 2

What % of O is NO? N:14 O:16 = 30 16/30 = 53.3 % O

A nitride of Cr is 84.8 %. It has a mass of ~ 184 g. What is the formula? = 15.2 % N 84.8 g Cr x 1 mol Cr = 1.63/1.09 = g Cr 15.2 g N x 1 mol N = 1.09/1.09 = g N Cr 1.5 N = / = 1.99 (2) (Cr 1.5 N) 2 = Cr 3 N 2