Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School





1)    How many moles of water are present in 13.5 g of water? Ans: 0.75

2)    How many grams of ammonia are present in 0.85 moles of the compound? Ans: 14.45

3)    Which is the molar mass of a substance, if a sample containing 3.2 moles has a mass of 313.6 g. Ans: 98

4)    How many moles of water are present in 24.95 g of copper sulfate pentahydrate? Ans: 0.5

(CuSO4•5H2O) (use the following Ar: Cu:63.5)

5)    A sample of MgSO4•XH2O is heated and it loses its water. An original mass of the hydrate had a mass of 36.945 g and after heating the mass is reduced to 18.045g

a) Calculate the mass of water present in the sample. Ans: 18.9
b) Calculate the moles of water present in “a”. Ans: 1.05
c) Calculate the moles of anhydrous compound in the heated sample (18.045 g) Ans:0.15
d) Divide the numbers in “b” and “c” by the smallest number. (That will give you the mole ratio between the heated sample and the water contained. Ans: ratio = 1/7
e) Write down the formula of the hydrate replacing the “X” by the number found in “d”. Ans:  MgSO4•7H2O

Percent Composition

Find the percent compositions of all of the elements in the following compounds: (ANSWERS IN ITALICS

1)     CuBr2              %Cu: _28.41__________           %Br: __71.59_______

2)     NaOH              %Na: _57.50__________            %O: ___40.0________  %H: __2.50_______

3)    (NH4)3PO4         %N: __28.19_________            %O: __42.95_________  %H: __8.05_______

       %P: __20.81_________             %PO4: __63.76_______  % NH4: __36.24____

4)   CuSO4•5H2O     %Cu: _25.45_____                    %S:    12.83                           %O: ___57.71____
% H: _4.01______                 % H2O: ___36.07____          % CuSO4: ___63.93____

Empirical Formula- Molecular Formula

1)      Find empirical formulas.

a) 70.9 % potassium, 29.1 % sulfur. Find empirical formula Ans:  K2S

b) 28.6 % magnesium, 14.3 % carbon, and the rest is oxygen. Find empirical formula. Ans:  MgCO3

c) 75 g of a compound formed by calcium, sulfur and oxygen has the following masses:  17.625 g sulfur, 35.325 g oxygen. Find the empirical formula. Ans:  CaSO4

d) 42.9 % carbon, 57.1 % oxygen. Find empirical formula. Ans:  CO

2)     A 20g sample of a hydrate of nickel sulfate (NiSO4•XH2O) lost 9.63g of water when heated. Determine the hydrate  formula.
 Ans:  NiSO4•8H2O

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.
Ans: CuSO4•5H2O
Hint: Calculate the empirical formula of the “6.4g of Cu, 3.2g of S, and 6.4g of O” present. Then add up the masses to get the molar mass of the compound. Then calculate the empirical formula of the compound as we explained during class.  

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. Ans: MgO

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. Assume that 1 mole of ANY gas will occupy 22.4 liters at STP. Ans: CH2 and C2H4
Hint: To calculate the molar mass use a proportion between the masses and volume that it takes.

6)     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, Mr=234  Ans: C5H11O2N and C10H22O4N2

b) 33.18 percent C, 4.60 percent H, 29.49 percent O, 32.72 percent Cl, Mr=108.5 Ans: C3H5O2Cl and C3H5O2Cl

c) 45.71 percent C, 10.48 percent H, 30.48 percent O, 13.3 percent N, Mr=210  Ans: C4H11O2N and C8H22O4N2

d) 40.00 percent C, 6.67 percent H, 53.33 percent O, MW=330 Ans: CH2O and C11H22O11

7) A 10g sample of a compound contains 4.00g C, 0.667g H, and 5.33g O. Find the empirical and molecular formulas. The Mr is 180 amu.  Ans: CH2O and C6H12O6