Problems on Chapter 4

V25.0109 General Chemistry I Honors

TA: Vaishnav Krishnan

vk276@nyu.edu

 

NAME: ______________________

 

 

 

 

 

1.                    Last week you balanced the following reactions.

Now, assign oxidation states to every atom in the reaction.

 

Ca3(PO4)2   +    SiO2   +    C     ŕ   CaSiO3     +    P4   +   CO 

 

Cu(NH3)4Cl2    +    NH3   +     Cu   ŕ     Cu(NH3)4Cl

 

 

Would the oxidation state depend on whether you balanced the chemical reaction correctly?  Does an atom’s oxidation state depend on the physical state of its molecule? If atoms and molecules are so stupid, then for those atoms in the so-called ‘Transition block’, what determines the oxidation state that they possess in a given compound?

 

 

 

 

 

2.                    Balance the following reactions using a method that involves half reactions. The medium in which the reaction proceeds has been provided in parentheses. Identify the reducing and oxidizing agents in the reactions.

a.                    Cr2­O72-  + CH3OH ŕ HCO2H + Cr3+ (acidic)

 

 

 

 

 

 

 

 

b.                    As + ClO3-1 ŕ H3AsO3 + HClO (basic)

 

 

 

 

 

 

 

 

3.                    A 6.977g sample of a mixture was analyzed for barium ion by adding a small excess of sulfuric acid to an aqueous solution of the sample. The resultant reaction produced a precipitate of barium sulfate, which was collected by filtration, washed dried and weighed. If 0.4123g of BaSO4 was obtained, what was the mass percentage of barium in the sample?

(Question within a question: How could you positively identify the precipitate as actually being barium sulfate?)

 

 

 

 

 

 

 

 

 

 

4.                    Concentrated phosphoric acid as sold for use in the laboratory is usually 90% H3PO4 by mass (the rest is water). Such a solution contains 12.2 mol of H3PO4 per liter of solution at 25C.

a.                    Compute the density of the solution

b.                    What volume of this solution should be used to in mixing 2.00L of a 1.00M phosphoric acid solution?

c.                    What volume of the original solution (laboratory stock solution) would be needed to titrate 25ml of a 0.7654M solution of barium hydroxide?

 

 

 

 

 

 

 

 

 

 

 

5.                    Goes on and on and on and on. . . Suppose 150ml of a 10% by mass solution of sodium chloride is acidified with sulfuric acid and then treated with an excess of MnO2(s). Under these conditions, all of the chlorine is liberated as diatomic chlorine gas. The chlorine is collected without loss and reacts with excess H2 to form HCl(g). The HCl is dissolved in enough water to make 250ml of solution. 50ml of this solution is used to titrate a 1.02M solution of NaOH. What volume of the NaOH solution would be required for this titration?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6.                    Cerium (IV) ions are strong oxidizing agents in acidic solution, oxidizing arsenious acid to arsenic acid according to the equation.

 

Ce4+ + H3AsO3 + H2O ŕ  Ce3+ + H3AsO4 + H+  

 

A sample of As2O3 weighing 0.217g is dissolved in basic solution and then acidified to make H3AsO3. Its titration with a solution of acidic cerium (IV)  sulfate requires 21.47ml. Determine the original concentration of Ce4+ in the titrating solution. Remember to balance this reaction.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7.        Explain the following in terms of energy changes.

a.        The exothermic dissolution of a salt in water

b.        The endothermic dissolution of a salt in water

c.        The reason for the high insolubility of a particularly insoluble salt like barium sulfate

 

 

 

8.        The Honors Super Challenge (sorry, all of my other problems on solution stoichiometry were pretty pathetic, so here is a better one on molecular geometry)

 

The molecule NH3 (ammonia) has been known to have a tetrahedral structure, with the electron pair pointing towards one vertex of the tetrahedron, and the N-H bonds pointing towards the other three. It is of course a ‘distorted tetrahedron’ with the HNH angle found to be not 109.5degrees but instead 107degrees. If we scale the approximate the length of the N-H bonds to be 1 unit long, and we position the N atom in NH3 to sit exactly at the origin of a three dimensional coordinate system, then calculate the three dimensional coordinates for each of the hydrogen atoms in the molecule, if one of the N-H bonds is aligned along the x-axis.

(Just so you know, you can conquer half of organic chemistry if you can easily visualize the three dimensional structure of an organic molecule. You need to be premed to conquer the other half.)