Which of the following statements best describes why a liter of mercury weighs more than a liter of water

Density of any substance can be measured using the following rule:
density = mass / volume

Since in this question, the volume of mercury is equal to the volume of water but still the mass of mercury is greater than the mass of water. 
Therefore, by substituting in the density equation, we will conclude that mercury is higher density than water.

The answer is:
Mercury is denser than water

Since there is the same amount of agn03 in both silver chloride and silver nitrate they produce the same amount of moles. therefore silver chloride produces the same amount of silver nitrate, so your answer is 15.0 moles of silver nitrate. i hope this !