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Percent composition, empirical formulas, molecular formulas,and formula for a hydrate

Percent composition, empirical formulas, molecular formulas,and formula for a hydrate. Percent Composition. Part/whole X 100 = % composition Ex. What percent of sodium chloride is chlorine? Chlorine is the “part” and sodium chloride is the “whole”

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Percent composition, empirical formulas, molecular formulas,and formula for a hydrate

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  1. Percent composition, empirical formulas, molecular formulas,and formula for a hydrate

  2. Percent Composition • Part/whole X 100 = % composition • Ex. What percent of sodium chloride is chlorine? • Chlorine is the “part” and sodium chloride is the “whole” • Mass of chlorine 35.45 g/mol = .6066 X 100 = 60.66% • Mass of NaCl 58.44 g/ mol

  3. Empirical formulas • Assume 100g of compound if given % • Convert % to g (it is 1 to 1) • Convert g to mole – round to 4 decimal places • Divide all by the smallest mol value • If within .1 of a whole number, round and use as subscripts • If not within .1 of a whole number, multiply by a number so that all are whole and use as subscripts. Ex. 1.5 X 2 =3 1.33 X 3 = 3.99 ~ 4

  4. Empirical Formulas from mass • If given mass (g) of each component element, convert them to moles. • Divide all by small • If within .1 of a whole number round and use a subscripts • If not within .1, multiply by “some number” to get a whole number(steps 2,3, and 4 are same as the previous slide) • Base sig figs on the masses given in the problem

  5. example • The composition of acetic acid is 40.00% carbon, 6.71 % hydrogen, and 53.29% oxygen.

  6. Molecular formulas • Determine empirical formula if it is not given • Determine empirical formula mass • Divide molar mass (a given) by empirical mass (should be within .1 of a whole #) • Multiply subscripts in the empirical formula by the quotient from # 3 to get subscripts for the molecular formula Ex. CH3O = 31.04 g/mol molar mass = 62.0 g/mol 62.0 g/mol = 1.997 ~ 2 C2H6O2 31.04 g/mol

  7. Formula of a hydrate • Remember that hydrates are compounds (especially ionic compounds) that have water molecules loosely attached to their crystals • Ex. CuSO4• 5H2O • If given % composition, assume 100 g of substance • Convert % to grams • Determine moles of both compound and water separately. • Divide by the smallest # of moles to get the mole ratio

  8. Example (works a lot like empirical formula) • Aluminum bromide is composed of 71.16% AlBr3 and 28.84% H2O • 71.16% = 71.16 g AlBr3 1mol = .2668 mol AlBr3 = 266.68g .2668 mol 1 • 28.84% = 28.84 g H2O 1mol = 1.601 mol H2O = 18.02 g .2668 mol 6.001 ~ 6 (within a 0.1) so the mole ratio is 6 waters to 1 AlBr3 AlBr3•6H2O

  9. Hydrates with Mass • If given “before” and “after” masses, determine how many grams of anhydrous (without the water) compound and how many grams of water then convert to moles and follow steps given. • If given the formula, determine what percentage is water and use that to determine the mass of anhydrous compound.

  10. One for you to try! • Cerium (III) iodide occurs as a hydrate with the composition 76.3% CeI3 and 23.7% water. Determine the formula for the hydrate.

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