Lessons Learnt From PH Calculations

Where did I make a mistake?

Initially, I didn't convert the volume from cm³ to dm³. That actually gave me a mole of 1.5 and 4 for the acid and base respectively. And when I plugged in the value during the pH calculation, I didn't quite get the desired result. More so, after rectifying that, I was about to go crazy when I didn't get the answer again even after converting the volume to dm³ via division by a thousand. I left the work for almost an hour before coming back to it.

So basically I continued to solve this question for about 30 minutes — a question that I was meant to solve in 2 minutes. I kept on getting an answer of 4 instead of 1.88. I wondered, where did I make the mistake?

I solved and solved... Not until I detected that I was meant to use the whole volume and not the subtraction of volumes. You know, normally after finding the mole of residual acid not used, one will want to also subtract the volume; however, that was the trap leading me astray.

Finally, I added the whole volume and I did get the final answer: 1.88. I was almost in tears when I saw that the answer was just nearby.

One thing to note is never to use the formula CₐVₐ/nₐ = CᵦVᵦ/nᵦ.

Additionally, since it was pH, and the concentration and volume were not the same, there would be an excess of either the acid or base. For the neutralization reaction you can, but for the pH it is advisable not to.

The first question was pretty easy — no stress, no hard work. I was able to achieve this within a period of 2 minutes. No need for conversion of units, and this made the question pretty straightforward.

However, the second question was tougher.

Thanks @stemsocial and @ecency for your constant encouragement.

Byeee!