Achieve question #7

[Xinyue Zou 2K]

Hello!
For question 7 on our achieve homework, how would we set up the balanced equation for this question in order to carry out the formula for Ka or Kb? How do we know which side we should put HClO and NaClO on and whether it makes H+ or OH-?
I've pased the problem below:
HClO is a weak acid ( Ka=4.0×10−8 ) and so the salt NaClO acts as a weak base. What is the pH of a solution that is 0.011 M in NaClO at 25°C?

[Carina H - 2G]

Hi!

For this problem, given the Ka, we are asked to find the pH of the weak base. So, we must find the Kb. The Kb is equal to 1x10^-14/Ka. Once you have the Kb, create an ice chart (assuming that OH- ions are made from the weak base) and then calculate X, and thus the [OH-]. Once you have this, find the pOH and then convert to pH.

Hope this helps!

[KPINTO 1B]

Hi! So for this one, because you are given a weak acid and its conjugate salt, we see that when NaClO dissolves, it turns into ClO- and when this reacts with water it creates OH, so we need to use the Kb rather than the Ka here. Hope this helps !

[VeronicaShepherd3B]

How does knowing a pH help reveal what the temperature is? What formula would we use?

[Alex FreeWolf 2E]

I believe pH decrease as temp increases but subtly (not to the point it goes from an acid to a base or vise versa). Not sure what equation though..

[Harsimer Bal 3K]

For #7, since Na is the conjugate acid of a strong base, its concentration does not change so it is not included in the equation. Thus, we are focusing solely on ClO- which is the conjugate base of a weak acid, HClO, so if we start with ClO- and place it in water, we know the products formed would be OH- and HClO- because H2O acts as an acid in this situation and donates a proton to ClO-. The question provides the Ka value but we are focused on a weak base in water so we need to find Kb using Kw/Ka=Kb. From there, we can solve for x and find the OH concentration and pOH and then find pH using pH=pOH=14.