19.1  Why are we not able to use the measured radial velocities of galaxies in the Local Group to evaluate the Hubble constant?

Because the Local Group is gravitationally bound together and the radial velocities of the galaxies are dominated by the gravitational interaction.

19.5  Why is Earth not expanding together with the rest of the universe?

Because it is held together by gravity, which is much stronger than the expansion of the universe over it's size.

19.7  Very distant galaxies have redshifts indicating recession speeds of 100,000 km/s or more, yet their true speeds are probably no more than a few hundred kilometers per second.  Explain.

The redshift of most galaxies is primarily due to the expansion of space itself, so most of what we see is not due to any motion of these galaxies.  Radial velocities due to gravity are generally about a few hundred km/s.

19.9  If you could accurately measure a triangle and a circle drawn on the surface of Earth, you would find that sum of the triangle's interior angles is more than 180 degrees and the circle's circumference is less than 2 x pi x r.  This is contrary to what you may have learned about triangles and circles in your geometry class.  Explain why this is so, and how it relates to a "spherical universe."

On a small part of Earth, Earth appears flat (like a plane).  In planar geometry, the angles in triangles add to 180 degrees and circles have circumferences of 2 x pi x r.   If you make a triangle or circle covering a lot of the Earth, the angles would sum to greater than 180 degrees and circles would be smaller than 2 x pi x r.  Similarly with a closed universe.  Over small regions of theunverse, a closed universe will appear planar.  Only very large triangles and circles would appear odd.

19.11  Hubble time (1/H) represents the age of a universe that has been expanding at a constant rate since the big bang.  Assuming a value of 23 km/s/Mly and a constant rate of expansion, calculate the age of the Universe in years.