Explain why methyl red would be a good indicator for a titration of 0.25M NaOH with .025M HNO3, but a poor choice for use in a titration of 0.25M NaOH with 0.25M HCN.
I'm pretty sure the first step is to find the pH of the two titrations (and then compare those to the pH range of methyl red), but I don't know (or can't remember) how..
To find pH for each equivalence point.
For HNO3 + NaOH ==> NaNO3 + H2O
That is a strong acid vs a strong base. Neither Na^+ nor NO3^- is hydrolyzed; therefore, the pH at the equivalence point is just that of pure H2O which is 7.0.
For NaOH + HCN ==> NaCN + H2O
The CN^- is a strong enough base to pull H^+ away from H2O; therefore, its pH is determined by the hydrolysis of th CN^-.
..........CN^- + HOH ==> HCN +OH^-
I........0.125............0....0
C.........-x..............x....x
E......0.125-x............x....x
Kb for CN^- = (Kw/Ka for HCN) = (x)(x)/(0.125-x). Solve for x = (OH^-) and convert to pH.
You can work this out but I obtained approx 11.
How did I arrive at 0.125M for CN^-? The titration of 0.25M HCN with 0.25M NaOH will require an equal volume of base added to whatever volume HCN was used initially so that dilutes th salt formed by a factor of 2.
Thanks :)
To determine why methyl red would be a good indicator for the titration of 0.25M NaOH with 0.025M HNO3 but a poor choice for the titration of 0.25M NaOH with 0.25M HCN, you are right in recognizing that we need to compare the pH ranges of methyl red with the pH values of the titration solutions.
Before finding the pH of the titration solutions, let's review the concept of a titration. A titration is a technique used to determine the concentration of a solution by reacting it with a solution of known concentration. In this case, we are determining the concentration of NaOH by reacting it with the known concentrations of either HNO3 or HCN.
To find the pH of the titration solutions, we first need to understand the reaction that occurs between the acid and base in each case.
1. Titration of 0.25M NaOH with 0.025M HNO3:
The reaction between NaOH (base) and HNO3 (acid) can be represented as follows:
NaOH + HNO3 → NaNO3 + H2O
Since NaOH is a strong base and HNO3 is a strong acid, the resulting solution will be neutral. Thus, the pH of the solution will be around 7.
2. Titration of 0.25M NaOH with 0.25M HCN:
The reaction between NaOH (base) and HCN (acid) can be represented as follows:
NaOH + HCN → NaCN + H2O
In this case, HCN is a weak acid. When it reacts with NaOH, it forms the weak conjugate base, CN-. The resulting solution will be slightly basic due to the presence of the weak base CN-. Thus, the pH of the solution will be greater than 7.
Now, let's look at the pH range of methyl red indicator. Methyl red is an acid-base indicator that changes its color based on the pH of the solution. It has a pH range between 4.8 and 6.0. Below pH 4.8, it is red, and above pH 6.0, it is yellow.
Comparing the pH values of the titration solutions:
1. Titration of 0.25M NaOH with 0.025M HNO3:
The pH of the resulting solution is around 7, which is outside the pH range of methyl red indicator. Therefore, methyl red would be a poor choice for this titration.
2. Titration of 0.25M NaOH with 0.25M HCN:
The pH of the resulting solution is greater than 7, which falls within the pH range of methyl red indicator. Therefore, methyl red would be a good indicator for this titration.
In summary, methyl red would be a good indicator for the titration of 0.25M NaOH with 0.25M HCN since the resulting solution falls within its pH range. However, methyl red would be a poor choice for the titration of 0.25M NaOH with 0.025M HNO3 as the resulting solution is outside of its pH range.