Scientists use scientific notation to simplify numbers, basically. When dealing with really big numbers or really small numbers, the usage of scientific notation prevents them from having to write a bunch of zeroes.
Hope that helped you!
Answer:
4.8 g/mL is the density of chloroform vapor at 1.00 atm and 298 K.
Explanation:
By ideal gas equation:

Number of moles (n)
can be written as: 
where, m = given mass
M = molar mass

where,
which is known as density of the gas
The relation becomes:
.....(1)
We are given:
M = molar mass of chloroform= 119.5 g/mol
R = Gas constant = 
T = temperature of the gas = 
P = pressure of the gas = 1.00 atm
Putting values in equation 1, we get:

4.8 g/mL is the density of chloroform vapor at 1.00 atm and 298 K.
Aluminum has three electrons in its valence shell. if you look at the periodic table, there are numbers at the top. in column 2, the elements have 2 valence electrons, in column 3 they have 3 valence electrons etc.