ANSWER KEY :   % Composition, Hydrate and Empirical & Molecular Formula Problems

Solve the following problems.  Show all of your work.  Attach extra paper if you need more space.


1.            Find the percent composition of a compound containing tin and chlorine if 18.35 grams of the compound contains 5.74 grams of tin.


            31.3 % Sn; 68.72 % Cl



2.            Calculate the percent composition of tetraphosphorus decoxide.



P4O10:     43.64% P; 56.36% O


3.            Calculate the percent composition of magnesium nitrate.



            Mg(NO3)2  16.39% Mg;  18.89% N;  64.72% O


4.            If magnesium hydroxide is composed of 41.7% Mg, calculate the number of grams of Mg in 185 grams of Mg(OH)2.


77.1 g Mg


5.            Which of the following compounds has the highest iron content?

               a)  FeO               b)  Fe2O3         c)  Fe3O4          A has highest Fe content

            a)  77.73% Fe;  b) 69.94% Fe;  c) 72.36% Fe


7.      Calculate the % by mass of water in calcium chloride hexahydrate (CaCl2 . 6H2O)


49.35 % water


8.      Calculate the % by mass of water in magnesium sulfate heptahydrate (MgSO4 . 7H20).


51.17% water


9.      What is the empirical formula for benzene, C6H6?    CH

10.  What is the empirical formula for butane, C4H10, commonly used in lighters?   C2H5


11.  Find the empirical formula of a compound that contains 75% carbon and 25% hydrogen.



12.  The most common form of nylon (Nylon-6) is 63.68% C, 12.38% N, 9.80% H and 14.14% O.  Calculate the empirical formula for Nylon-6.




13.  When a 0.3546 gram sample of vanadium metal is heated in air, it reacts with oxygen to achieve a final mass of 0.6330 gram.  Calculate the empirical formula of this vanadium oxide.





14.  Find the empirical formula for a hydrate of NiCl2 if it contains 45.4% water.


NiCl2 6 H2O



15.  Cisplatin, the common name for a platinum compound that is used to treat cancerous tumors, has the composition (mass percent) 65.02% platinum, 9.34% nitrogen, 2.02% hydrogen and 23.63% chlorine.  Calculate the empirical formula for cisplatin.






16.  What is the molecular formula of the molecule that has an empirical formula of CH2O and a molar mass of 120.12 g/mol?





17.  A compound is composed of 34.2% sodium, 17.7% carbon, and 47.6% oxygen.  Find its empirical formula.  If its molecular mass is 134 g/mol, find its molecular formula.


EF = NaCO2                Mol. F = Na2C2O4




18.  A compound used as an additive for gasoline to help prevent engine knock shows the following percentage composition:  71.65% Cl; 24.27% C; and 4.07 % H.  The molar mass is known to be 98.96 gram.  Determine the empirical formula and the molecular formula for this compound.



EF = CH2Cl                 Mol F = C2H4Cl2