Explanation:
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To rank the effective nuclear charge Z* experienced by a valence electrons of a set of atoms that belong to a same period, you only need to apply the rule of trend: it increases as you move from left to right in the period.
So, lets do it for these atoms: P, Al, Si, Cl
The belong to a same period and the order is Al, Si, P, Cl (just see a periodic table). So the rank is Al < Si < P < Cl
Now, lets do it for these atoms:, Be, Ne, O, C
They belong to the second period. The order is Be, C, O, Ne
So, the rank is Be < C < O < Ne.
HA ⇄ H⁺ + A⁻
so:
![\frac{[H^+][A^-]}{[HA]} = 1.5 x 10^{-5}](https://tex.z-dn.net/?f=%20%5Cfrac%7B%5BH%5E%2B%5D%5BA%5E-%5D%7D%7B%5BHA%5D%7D%20%3D%201.5%20x%2010%5E%7B-5%7D%20%20)
and now:

= 1.5 x 10⁻⁵
x is considered very small compared to 0.15
x² = 2.25 x 10⁻⁶
x = 1.5 x 10⁻³
So [H⁺] = 1.5 x 10⁻³
pH = - log [H⁺] = - log (1.5 x 10⁻³) = 2.83