If silver has a density of 10.5 ##"g/cm"^3## and gold has a density of 19.3 ##"g/cm"^3##, which would have the greater mass, 5 ##"cm"^3## of silver or 5 ##"cm"^3## of gold?

is defined as mass per unit of volume, which means that, for two substances that occupy the same volume, the more dense substance will weigh more.

In your case, silver has a of ##"10.5 g/cm"^3##, which means that ##"1 cm"^3## of silver weighs 10.5 g; likewise, gold's desnity is given as ##"19.3 g/cm"^3##, which means that ##"1 cm"^3## of gold weighs 19.3 g.

This means that, for equal volumes, the gold block will always weigh more than the silver block. The exact weighs of the two blocks will be

##5cancel("cm"^3) * "10.5 grams"/(cancel("1 cm"^3)) = "52.5 g"## ##->## the silver block

##5cancel("cm"^3) * "19.3 grams"/(cancel("1 cm"^3)) = "96.5 g"## ##->## the gold block

The gold block is heavier both because individual gold atoms are heavier than silver atoms, but also because they pack better - and that's what actually measures, how many atoms you can pack per unit of volume.