Chapter 27: 
Empirical & Molecular Formulas

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Questions:    1   2   3   4   5   6   7   8

1. Find empirical formulas for the compounds containing the following percentages of each element. Use these atomic weights: Li=7 Br=80 K=39 S=32 Mg=24 C=12 O=16 Ca=40 Sc=45

In all problems in which percentages of each element are given, because all the percentages of the elements in a compound add up to 100%, simply assume a 100g sample, then the weight of each element is the same as the percent. Calculate the moles of each element as: moles = grams/atomic weight.

a. 8.04 percent lithium, 91.96 percent bromide

b. 70.9 percent potassium, 29.1 percent sulfur

c. 28.6 percent magnesium, 14.3 percent carbon, 57.1 percent oxygen

d. 29.4 percent calcium, 23.5 percent sulfur, 47.1 percent oxygen

e. 65.2 percent scandium, 34.8 percent oxygen

f. 42.9 percent carbon, 57.1 percent oxygen

2. A 20g sample of a hydrate of nickel sulfate (NiSO4) lost 9.63g of water when heated. Determine the hydrate's formula.


3. A hydrate containing copper, sulfur, oxygen, and water lost 9g upon heating. Originally the hydrate had weighed 25g. Analysis of the anhydrous substance revealed that the 6.4g of Cu, 3.2g of S, and 6.4g of O were present. Find the formula of the hydrate.


4. When heated in the presence of oxygen, 12g of the magnesium forms and oxide weighing 20g. Find the empirical formula of the oxide formed.


5. A sample of gas occupies 2.0 liters at STP. The sample contains 2.143g of carbon and 0.358g of hydrogen. Find the empirical and molecular formulas of the gas.


6. A compound was found to contain 55.2 percent xenon and 44.8 percent chlorine. Find the empirical formula.


7. Find the empirical and molecular formulas for the compounds given below. Atomic weights: C=12 H=1 O=16 N=14 Cl=35.5

a. 51.28 percent C, 9.40 percent H, 27.35 percent O, 11.97 percent N, MW=234

b. 33.18 percent C, 4.60 percent H, 29.49 percent O, 32.72 percent Cl, MW=108.5

c. 45.71 percent C, 10.48 percent H, 30.48 percent O, 13.3 percent N, MW=210

d. 40.00 percent C, 6.67 percent H, 53.33 percent O, MW=330

8. A 10g sample of a compound contains 4.00g C, 0.667g H, and 5.33g O. Find the empirical and molecular formulas. The MW is 180 amu.


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