Answer : The amount of oxygen gas collected are, 0.217 mol
Explanation :
Using ideal gas equation :

where,
P = pressure of gas =
(1 atm = 760 torr)
V = volume of gas = 5 L
T = temperature of gas = 
n = number of moles of gas = ?
R = gas constant = 0.0821 L.atm/mol.K
Now put all the given values in the ideal gas equation, we get:


Thus, the amount of oxygen gas collected are, 0.217 mol
First gap is BY USING MAGNETS
2nd is ELECTROLYSIS
3rd is EVAPORATION
There are seven significant figures
Explanation:
If the density of the object is more than that of water, it will sink. Otherwise it will float. The density of water is 1 g/mL.
Substance 1,
Mass, m = 450 g, Volume, V = 90 mL
Density = mass/volume
So,

It will sink.
Substance 2,
Mass, m = 35 g, Volume, V = 70 mL
Density = mass/volume
So,

It will float.
Substance 3,
Mass, m = 24 g, Volume, V = 12 mL
Density = mass/volume
So,

It will sink.