Calculate the pH and the pOH of each of the following solutions at 25^∘ C for which the substanc

Calculate the pH and the pOH of each of the following...

Key Concepts

Stoichiometry of Ionization

In cases where compounds produce more than one hydrogen or hydroxide ion per formula unit, stoichiometric factors must be considered. For instance, a compound that releases two hydroxide ions per formula unit will have a hydroxide ion concentration that is twice its molar concentration, impacting the pH or pOH calculation accordingly.

Complete Ionization

The concept of complete ionization implies that when a strong acid or base dissolves in water, it dissociates entirely into its constituent ions. Thus, when calculating pH or pOH, one can assume that the concentration of ions directly equals the concentration of the substance, unless stoichiometric factors are involved.

Relationship Between pH and pOH

At 25°C, the sum of the pH and pOH of a solution is always 14. This relationship arises from the ion product of water (Kw = 1.0 x 10^-14), which links the concentrations of hydrogen and hydroxide ions in aqueous solutions.

pH and pOH Definitions

pH is defined as the negative logarithm (to base 10) of the hydrogen ion concentration in a solution, and pOH is similarly defined for the hydroxide ion concentration. They provide a measure of the acidity or basicity of a solution, with lower pH indicating stronger acidity and lower pOH indicating stronger basicity.

Strong Acids and Bases

Strong acids and bases are substances that ionize completely in aqueous solution. This complete ionization means that the concentration of hydronium ions (for acids) or hydroxide ions (for bases) is directly determined by the initial concentration of the acid or base.

Transcript

00:01 Problem 19 from chapter 14 is asking us to calculate the ph and poh of different solutions.

00:10 So to answer this question is important to remember that ph plus poh is always equal to 14.

00:18 Unless it's an incredibly strong acid or incredibly strong base, you're not going to see values above 14 or below 1.

00:27 So with that knowledge, we then need to be able to know how to calculate the ph and poh.

00:36 And to do so, we just take the negative log of the hydronium ions for ph.

00:43 And for poh, we are going to take the negative log of the hydroxide ion.

00:49 And then that will give us a poh.

00:52 So depending on which solution we are talking about, we are either going to first calculate.

00:58 Calculate ph and poh because it depends on the ions that the solution dissociates into.

01:06 So let's get started with hcl4.

01:12 And to answer this question, we are going to calculate the ph first.

01:18 And the reason being is it because this solution is going to dissociate into hydrogen ions, making it an acid.

01:26 So we are going to first calculate the ph and then we will calculate the poh.

01:33 So to calculate the ph, we are going to take the negative log of the molar concentration of the solution.

01:54 And when you go ahead and solve for that, what you will get is 3 .6.

02:03 So we have calculated the ph using the...

02:08 Malarity provided in the question because this will dissociate into equal parts c -l -o -4 and hydrogen ions.

02:17 So we can use this malarity to calculate the ph.

02:21 And then to calculate the ph, all we do is subtract our value for ph from 14 because as we can see up here, the ph plus poh is equal to 14.

02:38 So 14 minus the ph must give us the poh, and that is going to be 10 .4.

02:48 And this makes sense because the solution is an acid, it has a very low ph, denoting that it is an acidic solution.

02:59 So let's move on to the next part of the question, where we have 0 .21 moles per liter of sodium hydroxide.

03:09 So similar to the last question, we should analyze.

03:12 Which ions are going to dissociate from the solution.

03:15 And in this case, it's going to be the hydroxide ion.

03:19 So first we are going to have to calculate the p -o -h and then to calculate the ph, because hydronium ions are not going to dissociate in the solution.

03:28 So we must calculate p -o -h first.

03:31 So let's go ahead and solve for p -o -h, and we do that by taking the negative log of we provided polarity.

03:43 And when you do so, that gives us 0 .6.

03:53 Great.

03:54 And then now using this, we can calculate the ph.

04:00 Similar to what we did before, ph is going to equal, must equal 14 minus the value that we got for poh.

04:08 And in this case, it's going to be 13 .32...