[samtob]

I have the following question: Assume that you have two solutions. One of 50 cm3 HCl pH=1.3 and one of 150 cm3 Ba(OH)2 pH=12.3. If we mix these two solutions what will the pH of the mixture be?

[mikasaur]

Hello and welcome to the forums! Before we can help you you must show us that you've made an attempt on the question yourself.

What have you learned about pH? What concepts and formulas can you apply to this question?

[samtob]

I have done the following:
pH=1.3⇒[H+]=10−1.3=0.05… M⇒n=0.05…×0.05=0.0025…
pH=12.3⇒pOH=1.7⇒[OH−]=10−1.7=0.019…⇒n=0.019…×0.15=0.0029…
My problem is I do not know how to proceed

[mikasaur]

So you have the concentration of hydronium⁺ and hydroxide^- ions along with their actual "amounts".

What's going to happen when you mix those solutions and the hydronium and hydroxides get together?

[samtob]

I think I got it now:
The hydroxide and hydrogen ions will react and neutralize: 0,0029... - 0,0025... = 0,00048...
The concentration will be 0,00048/0,2 = 0,0024... => pOH = -log(0,0024...) = 2,61... => pH = 14 - 2,61... = 11,38

Is this correct?

[mikasaur]

Your approach looks correct. I didn't check any of the math so I'll trust you on that.

One thing you can do is what we in engineering call a sanity check (though I bet chemists do it too ).

Does your final answer make sense? Would you expect the mixing of those two solutions to produce a pH of something around 11.38?

[Borek]

sanity check (though I bet chemists do it too )

All the time.

Around 11.4 looks OK to me.

[samtob]

Could someone please check my answer. This is really important to me   

[mikasaur]

I quickly checked the math and got pretty much the same thing (sig figs aren't my strong suit).

[samtob]

Thank you!