Download CHEM 121 Rec Activity ANSWER KEY

Recitation Activity 1 (Chem 121)
Chapters 1 & 2
Significant Figures & Scientific Notation
1. How many significant figures are there in the following numbers:
a. 75.1
3 s.f.
b. 7.510 × 101
4 s.f.
c. 0.03400
4 s.f.
d. 125,000
at least 3 s.f.
2. Carry out the following calculations and express the answer using the correct number of
significant figures. Use scientific notation as appropriate.
a.
186.25 + 3.2 = 189.45 ? 189.5 (4 s.f.)
b. 661/0.0021 = 314761.9 ? 310000 ? 3.1 x 105 (2 s.f.)
c.
(44)(0.034) + (5.226)(2.2511) = 13.26 ? 13.3 (3 s.f.)
3. What’s the point of significant figures anyway. Consider the following example from the world
of finance. Let’s say you took $5,000 (or $5.000 × 103 to be precise) and invested it in the stock
market instead of using it on tuition (this would not have been a bad idea for some students in this
class, statistically speaking of course). If you bought Hitachi stock, which is very volatile right now,
at its rolling year long low ($40.95) and sold at its 52 week high ($81.35), how much money would
you have made? Now, if your broker lied to you and said that stocks prices were only recorded to
two significant figures, you would have lost how much of that amount?
If you invested $5,000 in the stock market you could buy 122.1 or 122 stocks. As a profit
you would make ($81.35-$40.95) or $40.40 per stock. Multiply $40.40/stock x 122 stocks
and the total profit ends up being $4928.80.
Now if your stock broker only uses two s.f. and you invested $5000 and bought each stock
for $41, you could buy 121.95 or 121 stocks. As a profit you would make ($81-$41) or $40
per stock. Multiply $40/stock x 122 stocks and the total profit ends up being $4880.
Take $4928.80 and subtract $4880 and your stock broker just took home $48.80.
This does not seem like a large amount of money, but you only invested $5,000 in the market.
**Please keep in mind that there are various ways to do this problem based upon how you
interpret it.**
4. We often feel like the magnitudes of numbers we use in chemistry are beyond our comprehension
(like 6.02 x 1023 ). The money spent by the federal government is likewise often unfathomable, but
we use words not scientific notation or prefixes to denote these values. What word does your
congressman use in his budget request to indicate
a. 106 or mega (M) dollars ? million
b. 109 or giga (G) dollars? ? billion
5. What are the values of the following federal government numbers in scientific notation?
From the OMB (Office of Management and Budget):
a. Federal Budget for 2004:
$2,272 billion ? $2,272 x 109 ? $2.272 x 1012
b. 2004 Federal Deficit:
$475 billion
c. Total, gross federal debt:
$7,586 billion ? $2,272 x 109 ? $7.586 x 1012
d. Debt held by the public:
$4,473 billion ? $2,272 x 109 ? $4.473 x 1012
? $475 x 109 ? $4.75 x 1011
Dimensional Analysis
6. In the movie Raiders of the Lost Ark, Indiana Jones replaces a golden statue with a bag of sand
of approximately the same size to compensate for its removal. Because the mass of the sand is
very different from the mass of the statue it springs a trap and a big stone sphere rolls out to
crush him. Let’s say that the statue had a volume of 1 Liter and it was made of pure gold (density =
19.3 g/mL ) How many bags of sand should he have put on the pedestal to compensate for the
statue? Assume the sand bags are also 1 Liter in volume and the density of sand is 2.3 g/mL.
1L x
1000 mL 10.3 g
1 mL
1L
1 bag
x
= 19,300 g x
x
= 8.4 L x
= 8.4 bags
1L
1 mL
2.3 g 1000 mL
1L
Atomic Structure
7. Complete the following table.
Symbol
18
O-
64
Zn
86
Sr2+
Protons
8
30
38
Neutrons
10
34
48
Electrons
9
30
36
Net charge
-1
0
+2
8. If you look at the periodic table in your book you will find that sodium (Na) has an atomic mass of
22.98770 amu, whereas lead (Pb) has an atomic mass of 207.2 amu. Why does the atomic mass of
sodium have so many more significant figures?
Na only has one isotope while Pb has 4 isotopes. The mass of an isotope is defined well out to
many sig figs, whereas the % abundance is only good to about a tenth of a percentage point.
9. You are given a box containing exactly 100 atoms of a single isotope. First you weigh the atoms
and find they have a total mass of 1.43 × 10-20 g. Next you react them with a box of fluorine ato ms
and find that 200 atoms of fluorine are consumed in the reaction (and the product is a tiny white
crystal). What is the chemical symbol for the isotope in question?
1.43 x 10-20 g /100 = 1.43 x 10-22 g/atom x 6.022x1023 atoms/mol = 86.1 g/mol
This MW is directly in between Rb and Sr. We are told though that the reaction of 100
atoms consumes 200 F atoms, giving a molecular formula of AF2 , making the A cation a 2+
cation and therefore it is 86 Sr.
Naming Compounds
10. There are two binary compounds that can form when the elements gold and iodine are reacted:
gold (I) iodide and gold (III) iodide.
a.
What is the empirical formula for gold (I) iodide?
AuI
b. What is the empirical formula for gold (III) iodide?
AuI3
c.
ionic
Are these compounds metallic, ionic or molecular?
d. If you heat either of the above compounds they will decompose back to form iodine and gold.
The iodine can then be washed away and you’re left with solid gold. If you were given a choice
of taking 1.0 kg of gold (I) iodide or 2.0 kg of gold (III) iodide (and your plan was to decompose
the compound and sell the gold for spending money), which would you take? If gold is worth $13
per gram how much money could you get by selling the gold?
2.0 kg AuI x
3
1000g 1 mol AuI
1 mol Au 197.05g Au
$13
x
x
x
= 682.05g Au x
= $8,867
1 kg 577.67 g AuI 1 mol AuI
1 mol Au
1 g Au
3
3
1.0 kg AuI x
3
1000g 1 mol AuI
1 mol Au 197.05 g Au
$13
x
x
= 608.27 g Au x
= $7,907
1 kg 323.87 g AuI 1 mol AuI
1 mol Au
1 g Au