Answer:
[H₃O⁺] = 3.162 × 10⁻⁹ moles / liter
Explanation:
The pH is calculated with the formula:
pH = -lg[H₃O⁺]
From here the concentration of hydronium ion (H₃O⁺) will be:
[H₃O⁺] = 
[H₃O⁺] = 
[H₃O⁺] = 3.162 × 10⁻⁹ moles / liter
We can use the dilution formula to find the volume of the diluted solution to be prepared
c1v1 = c2v2
Where c1 is concentration and v1 is volume of the concentrated solution
And c2 is concentration and v2 is volume of the diluted solution to be prepared
Substituting the values in the equation
15 M x 25 mL = 3 M x v2
v2 = 125 mL
The 25 mL concentrated solution should be diluted with distilled water upto 125 mL to make a 3 M solution

or simply 2,8 it is isoelectronic with argon
Number of proton K=19
so, 42 - 19 =23
then the answer in 19 protons and 23 neutrons