P < Q because 7.8 < 19.3
The density of the object does not change regardless of how much material the object has. A 100 gram block of iron will have the same density as a 200 gram block of iron. The density of iron will stay at 7.8 grams per cm^3. The same idea applies to the silver.
The density measures how packed a material is. The larger the density, the more molecules occupy the certain amount of volume.
A similar term is "population density" which measures how many people live in a certain area. It measures how packed a place is. A rural town will have low population density, while a big city has a high population density.