Explanation:
Let us assume that the concentration of [
and 
is equal to x. Then expression for 
for the given reaction is as follows.
![K_{w} = [OH^{-}][H^{+}]](https://tex.z-dn.net/?f=K_%7Bw%7D%20%3D%20%5BOH%5E%7B-%7D%5D%5BH%5E%7B%2B%7D%5D)
Now, we will take square root on both the sides as follows.
![[H^{+}] = 8.2 \times 10^{-8}](https://tex.z-dn.net/?f=%5BH%5E%7B%2B%7D%5D%20%3D%208.2%20%5Ctimes%2010%5E%7B-8%7D)
M
Thus, we can conclude that the 
concentration in neutral water at this temperature is 
M.
The answer is (3). The powdered iron and the piece iron is the same substance and is just different in the shape. So the reason is the power has larger surface area for reaction occurs.
It shows the atomic number
