The correct answer would be the last one.
Answer:
sample B contains the larger density
Explanation:
Given;
volume of sample A, V = 300 mL = 0.3 L
Molarity of sample A, C = 1 M
volume of sample B, V = 145 mL = 0.145 L
Molarity of sample B, C = 1.5 M
molecular mass of sodium chloride, Nacl = 23 + 35.5 = 58.5 g/mol
Molarity is given as;

The reacting mass for sample A = 0.3mol x 58.5 g/mol = 17.55 g
The reacting mass for sample B = 0.2175 mol x 58.5 g/mol = 12.72 g
The density of sample A 
The density of sample B 
Therefore, sample B contains the larger density
Answer:
4) 4Fe + 3O₂ → 2Fe₂O₃
Explanation:
4Fe + 3O₂ → 2Fe₂O₃
In this equation the numbers of atoms are same in both side. There are four iron and six oxygen atoms are present on left and right side of equation. That's why atoms are conserved. This equation completely followed the law of conservation of mass.
Law of conservation of mass:
According to the law of conservation mass, mass can neither be created nor destroyed in a chemical equation.
This law was given by french chemist Antoine Lavoisier in 1789. According to this law mass of reactant and mass of product must be equal, because masses are not created or destroyed in a chemical reaction.