Answer : The structure are shown below and the number of stereoisomers are possible in 3,5-dimethylnonane and 3,7-dimethyle-5-ethyldecane are 4 and 8 respectively.
Explanation :
Chiral carbon : It is carbon which is attached to the different types of atoms or molecules.
The number of stereoisomers possible for a compound is related to the number of chiral centers (n) present in it as,

where,
'n' is the number of chiral carbon present in the compound.
The number of chiral carbon present in 3,5-dimethylnonane is, 2 that means n = 2 (carbon-3 and carbon-5)

The number of stereoisomers are possible in 3,5-dimethylnonane is, 4
The number of chiral carbon present in 3,7-dimethyle-5-ethyldecane is, 3 that means n = 3 (carbon-3, carbon-5 and carbon-7)

The number of stereoisomers are possible in 3,7-dimethyle-5-ethyldecane is, 8
Answer:
8 Silicon atom are present in unit cell.
16 oxygen atoms are present unit cell.
Explanation:
Number of atoms in unit cell = Z =?
Density of silica = tex]2.32 g/cm^3[/tex]
Edge length of cubic unit cell = a = 0.700 nm = 

Molar mass of Silica = 
Formula used :
where,
= density
Z = number of atom in unit cell
M = atomic mass
= Avogadro's number
a = edge length of unit cell
On substituting all the given values , we will get the value of 'a'.
1 silicon is 2 oxygen atoms. then 8 silicon atoms will be 16 oxygen atoms.
First question

Balaneced:

Find how many moles potassium iodide it is in 345.0 grams

Find how many moles potassium nitrate there is for 2.078 moles 

Find how many grams potassium nitrate there is in 2.078 moles 

Second question

Half-life is the time required for the amount of something to fall to half its initial value. This term is usually used in nuclear physics to describe how quickly unstable atoms undergo decay, or how long stable atoms survive, radioactive decay, and it is also used more generally of any type of exponential or non-exponential decay.
In simpler terms: this is when an isotopes radioactivity is cut in half