Amixture is made by mixing 50.0 ml of 0.200 m hcl and 50.0 ml of water. what is its ph?

HCl is a strong base therefore it completely dissociates to H⁺ and Cl⁻ ions.
To calculate pH we need to know [H⁺]
number of HCl moles in 50.0 mL - 0.200 M /1000 mL/L x 50.0 mL = 0.01 mol
concentration of HCl - 0.01 mol/0.1 L = 0.1 M
[HCl] = [H⁺] due to complete dissociation
pH = -log [H⁺]
pH = -log (0.1 M)
pH = 1
pH of the solution is 1

And a) oxygen i believe