1.  This is Mrs. Jones. Her son won the scholarship last year. WHOSE

This is Mrs. Jones WHOSE son won the scholarship last year.

2.  I was sitting in a chair. It suddenly collapsed. WHiCH

The chair WHiCH I was sitting on suddenly collapsed or The chair on WHiCH I was sitting suddenly collapsed.

3.  This is the story of a man. His wife suddenly loses her memory. WHOSE

This is the story of a man WHOSE wife suddenly loses her memory. 

4.  I saw several houses. Most of them were really old. WHiCH

I saw several houses. most of WHICH were really old. 

5.  The Charlestons were given rooms in the hotel. Their new apartment had been destroyed in the explosion. WHOSE  

The Charlestons, WHOSE new apartment had been destroyed in the explosion, were given rooms in the hotel.

6.  The horse kept stopping to eat grass. I was on the horse. WHiCH

The horse WHICH I was on kept stopping to eat grass.