A 40.0 mL sample of a mixture of HCl and H3PO4 was titrated with 0.100 M NaOH. The first equivalence point was reached after 88.0 mL of base, and the second equivalence point was reached after 126.4 mL of base.

(e) Sketch the pH titration curve, and label the buffer regions and equivalence points.

Video transcript

Welcome back everyone. A solution containing HCL and H three A so four was prepared a 50 mL sample of this solution was titrated using 0.100 molar sodium hydroxide. The 1st and 2nd equivalent points were reached after the addition of 125 mL and 175 mL of the base respectively. Which of the following is the approximate pht ration curve for this scenario, we are given or titration curves and we want to determine which one of them represents this scenario. First of all, we have to understand that the starting solution contains two assets assets. We are essentially starting at a low ph. So the initial PH must be low and it must be increasing because we are adding a base that neutralizes the two assets. So our curve must be going upwards. What does that mean? Well, essentially we're going to exclude options A and B because those titration curves are going down, they start at high ph valleys close to 12 and they're going down. The PH is decreasing, meaning our solution gets more acidic and not more basic. We're expecting the ph to go up because we are adding a base, right? And the PH therefore must be increasing. So we are excluding options A and B because those scenarios show an increase in the, of each solution and not a decrease. So let's focus on C and D. First of all, they do represent a low initial P close to one, right. And the PH is increasing, which is consistent with the fact that we are adding a base. Now let's consider the equivalence points. And let's recall that equivalence points are the points where we have a drastic change in the P value, right? So essentially, they always belong to the vertical part of the curve and they're approximately in the middle. So if we look at our equivalent points considering titration curve D, those equivalence points are on the horizontal part of the curve. While the equivalent points for chart C, they are on the vertical part of the curve which is consistent with the definition of an equivalent point. This is where we have neutralized our acidic protons, right? And we have a drastic change in the ph. So the equivalent point always belongs to the vertical part of the curve and not the horizontal part of the curve. What we also notice is is that the first equivalence point for chart C it matches the volume of a 125 mL and the second one matches 175 mL. Those are consistent with the values given to us in the problem. While looking at chard D, we have volumes of 150. And then if we consider second equivalence point shown to us, what do we have? Well, essentially it is at approximately 225 this is not consistent with the values given to us in the problem. And therefore, we can say that the correct answer to this problem would be titration curve. C Thank you for watching.