Calculate the pH of a buffer solution that is 0.20 M in HCN and 0.12 M in NaCN. Will the pH change if the solution is diluted by a factor of 2? Explain.

Video transcript

Hi everyone for this problem. It reads find the ph of a solution of 0.250 molar hydrogen fluoride and 0.200 molar potassium fluoride before and after dilution by a factor of five. The PK of hydrogen fluoride is 3.20. So the question that we want to answer here is two things we want to know the P. H. Before and after dilution. So we're going to have to ph values. And the important information we're given here is what we're working with. We have this many moles of hydrogen fluoride and this many moles of potassium fluoride. We're told that the P. K. A. Of hydrogen fluoride is 3.2. Okay. And were also asked about it in terms of being by a factor of five. Okay, so a solution of hydrogen fluoride and potassium fluoride is a buffer. So let's go ahead and define what this means. So by buffer this means it's composed of a weak acid and its conjugate base and here are weak acid. Is the hydrogen fluoride and the conjugate base is the fluoride and ion. Okay, so since the solution is a buffer we can use the Henderson Hasselbach equation to determine its P H. And that equation is P H. Is equal to P. K. A plus log of the concentration of conjugate base over the concentration of weak acid. That's our Henderson Hasselbach equation. So we know what the peak A is. Okay and we know what the concentrations are for our weak acid and conjugate base. So all we need to do is plug in. Let's go ahead and replace concentration of weak acid and conjugate base by what we're actually working with. So we have P. H. Is going to equal P. K. A. Plus the log of the conjugate base is fluoride and ion and our weak acid is hydrogen fluoride. Okay, so now we can plug in. So what we're going to get is P. H. Is equal to the P. K. Was given and that is 3.20. And this is going to be plus the log of the concentration of the conjugate base is given and that concentration is 0.20 moller. Let me write that a little clearer. So it's 0.20 moller over the concentration of the weak acid which is given and that is 0.250 molar. So we have everything we need. So let's solve for P. H. So when you plug this into our calculator, p H equals 3.10. But the question asked us to find the ph before and after dilution by a factor of five. Even after dilution by a factor of five, the ph of the buffer solution will still be 3. because the concentrations of the weak acid and conjugate base are both going to decrease by the same factor. So the the ratio will be the same. So in terms of our answer for this question, the ph before the ph of the buffer before and after dilution by a factor of five is 3.10. So let's write that out. So before dilution It's 3.10. And after dilution It is also 3.10. So we can go ahead and highlight these two and that is it for this problem. I hope this was helpful.