1^st Semester Exam Review Questions for Units 4 and 5 (with answers)

1.  Aluminum (molar mass = 26.98 g) and dibromine (molar mass = 159.81 g) react to form the compound dialuminum hexabromide (molar mass = 533.38 g) according to the following balanced equation. 

2 Al(s) +  3 Br₂(l) à Al₂Br₆(s)

A total of 9.00 g of aluminum are available for reaction.  Dibromine has a density of 3.10 g/mL.  Avogadro’s Number is 6.02 x 10²³.  Answer the following questions. 

A)     Determine how many moles of aluminum atoms are present.  (Answer; 0.334 mol Al)

B)     Determine the mass (g) of dialuminum hexabromide that forms if all of the aluminum reacts according to the balanced equation.  (Answer: 89.1 g Al₂Br₆)  

C)     Determine the volume (mL) of liquid dibromine that is needed to react with all of the aluminum according to the balanced equation.  (Answer: 25.8 mL)

2. A 60.5-gram sample of a mixture contains 55.0% carbon tetrachloride by mass.  No other compounds of carbon or chlorine are present in the mixture.  The molar mass of carbon is 12.01 g, the molar mass of chlorine is 35.453 g the molar mass of carbon tetrachloride is 153.8 g.  Avogadro’s Number is 6.02 x 10²³.  Answer the following questions.

A)     Calculate the mass (g) of carbon tetrachloride in the sample.  (Answer: 33.3 g CCl₄)

B)     Calculate how many moles of chlorine atoms are present in the sample.  (Answer: 0.865 mol Cl)

       C)     Calculate the mass (g) of carbon present in the sample.  (Answer: 2.60 g C) 

                   D)   Determine the number of molecules of carbon tetrachloride in the sample.  (1.30 x 10²³ molecules)

3. Summarize key differences between ionic and molecular compounds in terms of:

(a)    the nature of the bonds

(b)   the state of the compound at room temperature

(c)    the arrangement of the ions or atoms and molecules

(d)   properties of the compounds

4. Review naming, balancing equations and identifying reactions.

__2__ NaHCO₃ (s)  -----> ____  Na₂CO₃ (s)  + ____  H₂O(l)  + ____ CO₂(g)

____CH₄(g)  +  __2__O₂(g)   ----> __2__ H₂O(g)  + ____ CO₂(g)

__2__ NH₄NO₃(s)    -------> __2__ N₂(g)  + __4__ H₂O(g)  + ____ O₂(g)

 __2__ ZnS(s)  + __3__ O₂(g)  -----> __2__ ZnO(s)  + __2__ SO₂(g)

        magnesium hydroxide (aq) + ammonium phosphate (aq) ----> magnesium phosphate (s)  +  ammonia (g)  + water (l)

        3 Mg(OH)₂ (aq)  + 2(NH₄)₃PO₄(aq)  -->  Mg₃(PO₄)₂(s)  +  6NH₃(g)  +  6H₂O(l)

5. Oxygen gas can be produced by decomposing liquid hydrogen peroxide (H₂O₂) into water and oxygen.  12.5 g of H₂O₂ decompose completely.  Answer the following questions.

            a.)     Write a balanced equation for this reaction.     (Answer:  2H₂O₂ (aq)  --> 2 H₂O (l)  +  O₂(g))

b.)    Mass-Mass problem: Determine the number of grams of water produced by 100% decomposition.  (Answer: 6.62 g H₂O)

            c.)     Mass-Volume problem: Determine the number of liters of Oxygen gas produced by this decomposition, assuming this reaction takes place at STP.   (Answer: 4.12 L)