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Chemical Calculations and Empirical Formulas

This text provides step-by-step calculations for finding molarity, empirical formulas, and percent composition of compounds. It also includes examples and helpful links for further understanding.

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Chemical Calculations and Empirical Formulas

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  1. convert the following: (4.0 x 106) X 0.0040 = _____ Find the gfw of Al(NO3)3 How many moles are in 0.0426 grams of aluminum nitrate? If 0.852 grams of aluminum nitrate are dissolved in exactly 500 cm3, what is the molarity? ACT-prep question: Which chart would you use to answer the following question? Do the data support the hypothesis that frogs in small populations call more frequently than frogs in large populations: March 14, 2005 Quiz: 1. 1.6 x 104 2. Al = 27 amu, N = 14 amu 0 = 16 amu (27) + 3(14) + 9(16)=213 amu or 213 g/mole 3. 0.0426 g mole_Al(NO3)3 213 g 2.00 x 10-4 mole aluminum nitrate 4. 0.852 gAl(NO3)3 mole_Al(NO3)3_1000 cm3 500 cm3 213 g Al(NO3)3 1 dm3 0.00800 M Al(NO3)3 note, .994 ~ 1

  2. mass in grams [concentration] ? M X dm3  GFW Moles  dm3 x GFW

  3. To Find % compositionfind gfwDivide mass of each partby the whole gfw. Ca3(PO4)2 3 Ca 3(40.1) 2 P 2(31.0) 8 O 8(16.0) 3 Ca 120. 2 P 62.0 8 O 128 310 g/mole = 120. = 62.0 = 128 3 Ca 120.g/310.g (100%) = 38.7% 2 P 62.0g/ 310g (100%) = 20.0% 8 O 128g/310 g (100%) = 41.3 % The sum should be within 1% of 100%

  4. HW 18: #56 64 p210; Honor’s #103 a,b and 104 a, b p 218 • Empirical Formula, first introduced in Chapter 7, refers to the experimental data you obtain when trying to find out how much of each element is in a sample of a compound. • Basically, you do the opposite of the procedure to find % composition. • Use grams to find a ratio of part:whole, • convert to moles, • find a mole ratio. • Write an empirical formula

  5. .57 g of Magnesium burns in air. 0.96 g of magnesium oxide is measured after combustion is complete. What is the empirical formula of magnesium oxide? 0.57 g Mg 0.96 g Mg?O? ____g O find mole ratio Mg: O 0.57 g Mg 0.96 g Mg?O? 0.39 g O 0.57 g Mg 24.3 g/mole Mg 0.39 g O 16.0 g/moleO 0.0235 mole Mg 0.0244moles O = 1 mole Mg 0.0235 0.0235 = 1 mole O Underline important information Find the # moles of each element Subtract 0.96 – 0.57 to find g O List what you have, and what you want. MgO

  6. HW 17: #50-55 page 208; #101p218 • % = the proportion out of 100 pieces • If there were 100, how much would be … • In a package of M&Ms 15 were brown, 6 were yellow, and 4 were blue. What % were brown? • 15 + 6 + 4 = total number of M&Ms = 25 • 15/25 = the fraction which are brown. • 60% are brown 15 x 100% 25

  7. Find the % nitrogen in ammonia. • NH3 = ammonia N = 14.0 3H = 3 x 1.0 17.0 amu 100% 14.0 amu N 17.0 amu NH3 = 82.4% N in NH3 http://www.ausetute.com.au/percentc.html Tutorial for extra help

  8. H = 2(1.01) O = 16.0 H2O = 18.0 g/mole % H = 0.112 x 100% 11.2% H % O = 0.889 x 100% 88.9% O To find the % composition of H2O %H = 2.02 g H2 18.0 g/mole H2O %O = 16.0 g O 18.0 g/mole H2O

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