Calculate the ph of a mixture made by adding 50.0 ml of 0.20 m hcl(aq) to 150.0 ml of water if the temperature of the mixture is 25.0 °c.

PH scale is used to determine how acidic or basic a solution is,
pH can be calculated once the [H⁺] is known.
Here HCl is a strong acid therefore it completely dissociates into H⁺ ions and Cl⁻ ions.
Therefore [HCl] = [H⁺]
number of HCl moles added - 0.20 mol/L x 0.050 L = 0.010 mol
The volume of the final solution = 50.0 + 150.0 = 200.0 mL
then the concentration of HCl = 0.010 mol / 0.200 mL = 0.05 mol/L
therefore [H⁺] = 0.05 M
then pH can be calculated using the following formula,
pH = -log[H⁺]
pH = -log(0.05 M)
pH = 1.30

A.

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.