Chemistry 101

Recitation 9/5/03       Chapters 1 and 2

 

Your group should work each problem together then prepare to present your solution to the class.  Student #1—Introduce your group to the class.  Student #2—What is the problem?  Student #3—What is your strategy for solving the problem?  Student #4—Explain the math.  Student #5—Explain your use of significant figures.

 

  1. The density of gold is 19.32 g/mL.

A 10-mL graduate cylinder is filled to 5.00 mL. A ring is placed in the graduated cylinder, and the water level rises to 5.15 mL.  The ring is then dried and placed on a balance, and its mass is 2.8315 g. Find the density of the ring.  Is the ring pure gold?  Explain your reasoning.

 

Answers—To 2 significant figures, the density calculated, 2.8315 g / 0.15 mL is 19 g / mL.

To 2 sf the density of gold is 19 g / mL.  Yes, this could be gold.

 

  1. The atomic radius of gold is 144 pm and 1 angstrom = 10^ -10 m.  A single layer of gold atoms forms a surface whose dimensions are 1.0 x 10^3 angstroms by 1.0 x 10^3 angstroms.  What is the surface area in square centimeters?  How many gold atoms form this layer?  You may assume that the gold atoms touch each other.
  2.  

If 1 Å = 10^ -10 m then 1Å = 10^ -8 cm

Surface Area = 1000 Å (10 ^ -8 cm)/( 1 Å) x 1000Å (10 ^ -8 cm)/( 1 Å)   = 10 ^ -10 cm^2

Drawing a picture of the square surface filled with gold circles is very helpful!

1 Å = 100 pm

Consider 1 edge of the square: 1000 Å = 1000 x 100 pm = 10 ^ 5 pm.

The number of gold atoms that can fit along the edge is 10 ^ 5 pm / ( 2x 144 pm) = 347 atoms.  We have a square filled with gold circles and there are 347 along each edge or 347 x 347 = 1.2 x 10^3 atoms total.  The answer should be to 2 sfs.

 

  1. Consider the 23 Na atom.  Given that the radius and mass of the nucleus are 3.04 x 10^ -15 m and 3.82 x 10 ^ -23 g, respectively, calculate the density of the nucleus in g/cm^3.  The radius of a 23 Na atom is 186 pm.  Calculate the density of the space occupied by the electrons in the sodium atom.  Do your results support Rutherford’s model of the atom?  The volume of a sphere is 4/3 p r^3 and you many find values for fundamental constants inside the back cover of your textbook.

<IMG SRC="ruth1.gif" WIDTH=700 >

<IMG SRC="ruth2.gif" WIDTH=700 >

Hint:  You need to consider the mass of 11 sodium electrons!

 

 

4. The Table gives numbers of electrons, protons, and neutrons in atoms or ions of a number of elements.  A) which of the species  are neutral? B) Which are negatively charged? C) Which are positively charged? D) What are the conventional symbols for all the species?

Atom or ion

A

B

C

D

E

F

G

Number of protons

5

10

18

28

36

5

9

Number of electrons

5

7

19

30

35

5

9

Number of neutrons

5

7

20

36

46

6

10

<IMG SRC="isotope.gif" WIDTH=700 >

 

Your US Open predictions? Clijsters  Capriati            Davenport        Henin-Hardenne

  Agassi           Roddick           Hewitt  Nalbandian El Aynaoui

Have a great “Owls on the Prowl” weekend !