Simple dimensional analysis. 
okay so youll need a periodic table to look up the molar mass. youll be given either an amount of grams or moles. 
 
        
        
        
Answer:
D.
Explanation:
The reaction is losing potential energy, which means that the reaction is losing that energy as heat. Exothermic is the loss of energy. Therefore it will be D.
 
        
                    
             
        
        
        
Answer:
The molarity is 0.56
Explanation:
In a mixture, the chemical present in the greatest amount is called a solvent, while the other components are called solutes. Then, the molarity or molar concentration is the number of moles of solute per liter of solution.
In other words, molarity is the number of moles of solute that are dissolved in a given volume.
The Molarity of a solution is determined by:

Molarity is expressed in units ( ).
).
Then you must know the number of moles of Cu(NO₂)₂. For that it is necessary to know the molar mass. Being:
- 
Cu: 63.54 g/mol
- N: 14 g/mol
- O: 16 g/mol
the molar mass of Cu(NO₂)₂ is:
Cu(NO₂)₂= 63.54 g/mol + 2*(14 g/mol + 2* 16 g/mol)= 155.54 g/mol
Now the following rule of three applies: if 155.54 g are in 1 mole of the compound, 225 g in how many moles are they?

moles= 1.45 
So you know:
- number of moles of solute= 1.45 moles
- volume=2.59 L
Replacing in the definition of molarity:

Molarity= 0.56
<u><em>The molarity is 0.56</em></u> <u><em></em></u>
<u><em></em></u>
 
        
             
        
        
        
Answer:
74
Explanation:
we know,
atomic number = number of protons
so,
atomic number = 74