7) there are two ways to solve this.
---first method
lets calculate the [H+] concentration using the OH concentration, remember there is this formula--> [H+] x [OH-]= 1.0x10^-14,so solving for [H+] we get
[H+]= 1.0x10^-14/ [OH-]-----------> 1.0x10^-14/ 0.004= 2.5 x 10^-12
then, we find pH using this formula-->pH= -log {H+]
pH= -log (2.5 x 10^-12)= 11.6
------second method
we can first find pOH using the [OH-] concentration, remember that we have this formula--> pOH= -log [OH-]
pOH= -log 0.004= 2.4
then we find pH using this formula--> pH + pOH= 14, so
pH= 14- pOH-------> pH= 14 - 2.4= 11.6
so the answer is C
8)
first, since we already have the [H+] concentration, we can find pH using the formula: pH= -log [H+]
pH= -log 1.5 x 10^-4= 3.82
to find pOH, we use this formula: pH +pOH= 14
pOH= 14 - pH-----> pOH= 14 - 3.82= 10.2
so answer is A
