Answer:
White or gray, odorless lumps or granular powder.
Answer:
Number of neutrons: 44
Number of protons: 34
Number of electrons: 36
Explanation:
The atomic number of Se (Selenium) is 34
Mass number is 78
Number of neutrons = mass number - atomic number = 78 - 34 = 44
Number of protons is the same as the atomic number
Number of protons = 34
Selenium ion has a charge of -2. This means it accepts two more electrons
Number of electrons = atomic number + 2 = 34 + 2 = 36
The full question can be seen below:

The decomposition of
is represented by the equation above.
A student monitored the decomposition of a 1.0 L sample of
at a constant temperature of 300K and recorded the concentration of
as function of time. The results are given in the table below:
Time (s) 
0 2.7
200 2.1
400 1.7
600 1.4
The
produced from the decomposition of the 1.0 L sample of
is collected in a previously evacuated 10.0 L flask at 300 K. What is the approximate pressure in the flask after 400 s?
(For estimation purpose, assume that 1.0 mole of gas in 1.0 L exerts a pressure of 24 atm at 300 K).
Answer:
1.2 atm
Explanation:
Considering all assumptions as stated above;

Initial 2.7 mole --- ---
Change -1.0 --- 
Equilibrium 1.7 mole --- 0.5 mole
To determine the concentration of O₂; we need to convert the moles to concentration for O₂ = 
= 
= 0.05 
Thus, based on the assumption that "1.0 mole of gas in 1.0 L exerts a pressure of 24 atm"
∴ 0.05
will give rise to = 0.05
× 24
= 1.2 atm
1 micrometer is equal to 10⁻⁶m so the correct answer is <span>A.) 0.000001 meter</span>