In a displacement reaction, a more reactive element replaces a less reactive element from a compound. In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.
Explaination:
Displacement chemical reaction : 
A displacement reaction is the one wherein the atom or a set of atoms is displaced by another atom in a molecule. For instance, when iron is added to a copper sulphate solution, it displaces the copper metal.
Fe + CuSO4 → FeSO4 + Cu. When zinc is added to a copper sulphate solution, it displaces the copper metal. ...
Zn + CuSO4 → ZnSO4 + Cu.
etc. 
Double Displacement Chemical Reaction :
Double displacement reactions take place mostly in aqueous solutions wherein the ions precipitate and exchange of ions takes place. For example, when a solution of barium chloride is mixed with sodium sulphate, a white precipitate of barium sulphate is formed rapidly. These reactions are ionic in nature.
Here are some examples of double displacement reaction: AgNO 3 + NaCl → AgCl + NaNO.
 2NaCl + CaSO4 → Na2SO4+ CaCl.
H2SO4 + 2LiOH ⇌ Li2SO4 + 2H2O.
 
        
             
        
        
        
Answer:
star
Explanation:
stars are composed of 92.1 percent hydrogen and 7.8 percent helium.
 
        
             
        
        
        
5-Ethyl-3,3,4-trimethylheptane has 3 secondary carbon. A primary carbon written as 1° is a carbon that has one carbon atom attached to it. A secondary carbon written as 2° is a carbon attached to two other carbons, while a tertiary carbon written as 3° is a carbon attached to three other carbons. 
        
                    
             
        
        
        
Answer:
C
Explanation:
Alcohols are organic molecules characterized majorly by the presence of the OH group in their molecule. The OH group is majorly responsible for several of their characteristics. This include the formation of hydrogen bonds between alcohol molecules. While this makes them more inorganic than most organic compounds, comparatively the hydrogen bonding formed in alcohols is not as strong as that which is present in water.
The higher strength of the hydrogen bonding is responsible for some comparable properties. While water boils at a temperature of 100 degrees Celsius, alcohol boils at a temperature of 78 degrees Celsius. This is an evidence to the fact that hydrogen bonding in alcohol is less stronger that that in water.
 
        
             
        
        
        
Mole is mass (g) / Molar mass (mole/gram) 
So to find mass in gram multiply the no.mole by Molar mass