Assignment 2
7. Zinc phosphate is used as a dental cement. A 50.00 mg sample is broken down into
its constituent elements and gives 16.58 mg oxygen, 8.02 mg phosphous and 25.40
mg zinc. Determine the empirical formula of zinc phosphate.
Convert weight of each element into the number of moles of each element:
16.58 mg oxygen = 16.58/15.999 mmoles = 1.04 mmoles
8.02 mg phosphorous = 8.02/30.974 mmoles =0.259mmoles
25.40mg zinc = 25.40/65.407 = 0.388 mmoles
We divide all by the smallest number, that for phosphorous (thereby getting a formula
with one P in it),
P= 1
O = 1.04/0.259 = 4.02
Zn = 0.388/.259 = 1.5
Now multiply everybody by 2 to get integers: Zn3(PO4)2
19 3H2 + N2 = 2NH3
2K + O2 = K2O2
etc.
31. An element X has a dibromide with the empirical formula XBr2 and a dichloride with
empirical formula XCl2. The dibromide is completely converted to the dichloride
when it is heated in a stream of chlorine according to the equation
XBr2+ Cl2 = XCl2 + Br2.
When 1.500 g of XBr2 is treated, 0.890 g of XCl2 results.
a) calculate the atomic mass of X
according to the above chemical equation, one mole of XCl2 is produced for each mole
of XBr2 consumed.
The number of moles of XBr2 is wt/MW = 1.500/(X+2*79.90) = moles XCl2
=.890/(X+2*35.453)
So 1.5000*(X+2*35.453)=.890*(X+2*79.90)
(1.5000-.890)X = 2*79.90*.890-2*35.45*1.500
0.61X = 35.872
X=58.81
Could be Ni or Co
36. The poisonous gas HCN is produced by the high temperature reaction of ammonia
with methane. Hydrogen is also produced.
a) Write a balanced reaction
NH3+ + CH4 = HCN +3 H2
b) suppose 500.0 g CH4 is mixed with 200.0 g of NH3. Calculate the masses of the
substances present after the reaction is allowed to proceed to completion.
Exactly 500g of CH4 is unlikely to react with exactly 200g NH3 so it's likely something is
in excess. Calculate the number of moles of each reagent:
nCH4 = wt/mw = 500/16 = 31.25
nNH3= wt/mw = 200/17= 11.76
These gases react on a 1:1 basis, so all of the NH3 is used, leaving (31.25-11.76)=19.48
moles CH4 = 19.48*16=311.8 g CH4.
We have made 11.76 moles of HCN and 3*11.76 moles of H2 which weigh
11.76*27=317.5 g and 70.56 g respectively.
Check: everything should add up to 700g: 311.8+317.5 + 70.56 =699.86 (not 700
because of rounding errors)
52. A newspaper article about the danger of global warming from the accumulation of
greenhouse gases such as carbon dioxide states that "reducing driving your car by 20
miles a week would prevent release of over 1000 pounds of CO2 per year into the
atmosphere." Is this a reasonable statement?
Assume that gasoline is octane (C8H18) and that is burned completely to CO2 and H2O
in the engine of your car. Facts or guesses about your car's gas mileage, miles driven,
the density of octane and other factors will also be needed.
In order to see if this notion is reasonable we do a qualitative calculation. I take my car
as example. I get 27 mpg, so20 miles per week * 52 weeks/year requires 1040/27=38.5
gal of gasoline. This is about 4*38.5 l = 154 l. If the density is about 1 this is 154kg or
154/(8*12+18)kmoles = 1.35kmoles
The combustion reaction is
C8H18 +25/2 O2 = 8CO2 + 9H2O
So one mole of octane produces 8 moles of CO2, and this driving would produce 1.35*8
kmoles =10.8 kmoles CO2 or 10.8*46 kg = 497kg or 497/454 kpound = 1.1kpounds =
1100 pounds, which is about what the article claims!
Note that this works out to about 1 pound/mile!
Chapter 3
1. Scandium will have properties intermediate between Ca and Ti, with the properties
probably being closer to those of Ti since this is the transition metal series. In this
series the properties change relatively slowly. We might thus predict the mp to be ≈
1500, and the bp ≈ 3000 with a density ≈ 4.. The actual mp is 1541, so that guess is
pretty close. The bp is 2831, so we're close there. The density is 2.989 = 3, which is
lower than our guess.
5. For each of the following pairs of atoms, state which you expect to have the higher
first ionization energy.
a. Rb or Sr
Rb is an alkali metal and easily ionized, so Sr has the higer ip.
b. Po or Rn
Rn is arare gas so it will have a higher ip
c. Xe or Cs
Cs is an alkali so Xe should have the higher IP
d. Ba or Sr
both are alkaline earths, but Ba lies lower and will have the lower
ip
so Sr shold have the higher IP
9.
For each of the following pairs of atoms, state which you expect to have the greater
electron affinity
a) Xe or Cs.
Cs has the lowest IP known, so it doesn't want an electron
very much, but neither does Xe. An extra electron in Cs
completes the 6s sub-shell, but an extra electron in Xe would
have to go into the unoccupied 6s shell. Xe- doesn't exist! So
Cs has the higher EA
b) Pm or F
F has the higher EA. This is a sure bet.
c) Ca or K
Probably K because it would complete the 4s sub-shell. In
fact Ca- doesn't exist
d) Po or At
At is a halogen and would have a higher EA