7.4  Explain how the size and mass of the terrestrial planets, coupled with their distance form the Sun, have determined the kind of atmosphere that each (including the Moon) has today.

The size and mass of a planet determine its surface gravity, which determines what types of gases can exist in the atmosphere of the planet.  Distance from the Sun is the other part -- the closer to the Sun, the hotter the atmosphere, so the faster it moves and the more easily it escapes.  Mercury is close and small -- no atmosphere.  The Moon is too small.  Mars is far from the Sun, but it is only slightly larger then Mercury, so it can hold some atmosphere.  Venus and Earth are both massive enough to hold atmospheres.  Earth is far enough from the Sun to keep its atmosphere relatively cool, while Venus "boiled away" its carbon dioxide through a runaway greenhouse effect.

7.8  Why is Venus very hot and Mars very cold if both of their atmospheres are dominated by carbon dioxide, a good "greenhouse" gas?

Although the fraction of carbon dioxide in both atmospheres is high, Venus has a lot of atmosphere (and thus a lot of carbon dioxide) while Mars has very little atmosphere (and thus very little carbon dioxide).

7.11  Global warming appears to be responsible for increased melting of ice in Earth's polar regions.
           a.  Why does the melting of Arctic ice, which floats on the surface of the Arctic Ocean, not affect the level of the oceans?
           b.  What effect is the melting of glaciers in Greenland and Antarctica having on the level of the oceans?

a.  This ice is already in the water, and melting doesn't raise the level of the water.   You've certainly noticed this in a glass of ice water - even if it's full to the brim, it doesn't overflow when the ice melts.  

b.  On the other hand, the glaciers in Greenland and Antarctica are on land, so when they melt, the meltwater runs into the ocean and raises the level.  In fact, if all the ice on Greenland and Antarctica melted, the oceans would rise about 65 meters!