Question

A plastic bottle of mass 500g has a volume of 450cm3. Calculate the mass of water displaced by it if itsinks in water.

Answer 1

it will displace 450 grams of water .
 because water displaced = volume of the object.* density of water.
i hope that this will help u.

Answer 2

When there is a question to calculate the mass of water displaced, you first need to find the density of the object.
Here, mass = 500g
volume = 450 cm³
density = m/V = 500/450 = 1.11 g/cm³

Next, you need to compare the object's density with density of water. There are two cases possible. Density of water is 1g/cm³

Case-1: Density of object is equal to or more than 1g/cm³
If the density of the object is more than density of water, it will submerge in water. When it submerges, it will displace water equal to it's volume.

Case-2: Density of object is less than 1g/cm³
If the density of the object is more than density of water, it will float on water. 
When it floats, the ratio of volume submerged to total volume of object is given by:
$\frac{volume\ submerged}{total\ volume} = \frac{density of object}{density\ of water} =density\ of\ obect(in\ g/cm^3)$

Coming to the problem,
density of our object = 1.11 g/cm³ > 1g/cm³(case-1)
So it will submerge.
Volume of water displaced = volume of object = 450cm³
mass of water displaced = density×volume = 1×450 = 450 gram

Mass of water displaced is 450g.

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If you want a simple solution without explanation:
mass = 500g
volume = 450 cm³
density = m/V = 500/450 = 1.11 g/cm³
density of object is more than density of water. So it will submerge and displace same volume of water.
So volume of water displaced = 450cm³
mass of water displaced = density×volume = 1×450 = 450 gram

Mass of water displaced is 450g.