540 CHAPTER 13: Liquids and Solids
BUILDING YOUR KNOWLEDGE*122.More than 150 years ago Pierre Dulong and A. T. Petit
discovered a rule of thumbthat the heat capacity of one
mole of a pure solid element is about 6.0 calories per °C
(i.e., about 25 J/°C). A 100.2-g sample of an unknown
metal at 99.9°C is placed in 50.6 g of water at 24.8°C.
The temperature is 36.6°C when the system comes to
equilibrium. Assume all heat lost by the metal is absorbed
by the water. What is the likely identity of this metal?
*123.In a flask containing dry air and some liquidsilicon tetra-
chloride, SiCl 4 , the total pressure was 988 torr at 225°C.
Halving the volume of the flask increased the pressure to
1742 torr at constant temperature. What is the vapor
pressure of SiCl 4 in this flask at 225°C?
*124.A friend comes to you with this problem: “I looked up
the vapor pressure of water in a table; it is 26.7 torr at
300 K and 92,826 torr at 600 K. That means that the
vapor pressure increases by a factor of 3477 when the
absolute temperature doubles over this temperature
range. But I thought the pressure was proportional to the
absolute temperature, PnRT/V. The pressure doesn’t
just double. Why?” How would you help the friend?
*125.Using as many as six drawings (frames) for each, depictthe changes that occur at the molecular level during each
of the following physical changes: (a) melting an ice cube,
(b) sublimation of an ice cube below 4.6°C and 4.6 torr,
and (c) evaporation of a droplet of water at room tem-
perature and pressure.
*126.Write the Lewis formula of each member of each of the
following pairs. Then use VSEPR theory to predict the
geometry about each central atom, and describe any fea-
tures that lead you to decide which member of each pair
would have the lower boiling point. (a) CH 3 COOH and
HCOOCH 3 , (b) NHF 2 and BH 2 Cl, (c) CH 3 CH 2 OH and
CH 3 OCH 3.
*127.At its normal melting point of 271.3°C, solid bismuth has
a density of 9.73 g/cm^3 and liquid bismuth has a density
of 10.05 g/cm^3. A mixture of liquid and solid bismuth is
in equilibrium at 271.3°C. If the pressure were increased
from 1 atm to 10 atm, would more solid bismuth melt or
would more liquid bismuth freeze? What unusual prop-
erty does bismuth share with water?
*128.The doping of silicon with boron to an atomic concen-
tration of 0.0010% boron atoms vs. silicon atoms
increases its conductivity by a factor of 10^3 at room tem-
perature. How many atoms of boron would be needed to
dope 12.5 g of silicon? What mass of boron is this?