[demonat0r]

[Borek]

x is whatever you want to call x, it can be an average height of elephant.

This is a buffer question.

In this type of the question you usually assume that the reaction between weak acid/base and strond base/acid went to completion.

[demonat0r]

ok but what is the reasoning involved to decide that x in this problem represents the concentration of HCl?

[demonat0r]

help please

[Borek]

If you start with solution that has pH higher than you need, you have to add acid to lower pH. If you wan't higher pH, you have to add base. That's the way it works. Always.

[demonat0r]

[Borek]

Sorry, I have no idea what is confusing to you. You have to choose between HCl and NaOH first. You need to get pH lower, so you select HCl. Why you still have no idea how to decide you need to use HCl?

Then it is just down to the reaction stoichiometry. When you add strong acid to weak acid salt, strong acid protonates conjugate base of weak acid, converting it into acid. We assume this reaction goes to completion and calculate concentrations of both weak acid and its conjugate base from the stoichiometry.

[demonat0r]

[Borek]

I think I have answered your question several times already. Perhaps you should try to read my earlier answers again.

x is a variable, it represents whatever you want it to represent. Clearly in this case to lower solution pH you have to add some unknown amount of HCl. This unknown amount is your variable and it is denoted by x. It can be y, or z or wtf if you prefer, symbol doesn't matter.

Sorry, but I don't think I will be able to help you further.

[demonat0r]

so according to what you said, using the equation Kb = (.0833 + x)(x) / (.225 -x)
i can let x = anything i want?
can i let x = concentration of NaOH or concentration of HC2H3O2?

[Borek]

No, you select what you want it to be first, than you construct the equation. You can't build the equation first and decide what your variables mean after that.

And in case you will ask again the same question - you know that you have to add HCl becuse you are going to lower the pH. Thus you know you have to calculate amount of HCl, so you assign amount of HCl to variable x and you build the equation to calculate what happens when you add x moles of HCl. Then, knowing final pH, you calculate what vaue of x is needed.

[demonat0r]

oh ok
sry for the trouble, just trying to understand this
i understand that you need to add HCl to the solution but,
i don't understand how to construct the equation that lets you solve for concentration of HCl

[Borek]

I have explained that several times. Stoichiometry.

[demonat0r]

sry all you have said was use stoichiometry and i don't understand how to construct the equation.
so is this the equation formed with stoichiometry?
Kb = (.0833 + x)(x) / (.225 -x)

doesn't the strong acid react with the weak base before the system reaches equilibrium? so in addition to those values .833 + x and .225 - x, shouldn't there be more changes in concentrations to reach equilibrium?

[demonat0r]

would anyone be kind enough to tell me how to get the formula to use for this problem?