Titrations and Indicators - AP Chemistry

The equivalence point for a strong-acid / weak-base titration will be at a slightly acidic pH. This is because the acid is stronger and dissociates to a greater degree, while the base is not quite as strong, so doesn't dissociate to a large enough extent to neutralize each equivalent of the acid.

The equivalence point for a weak-acid / strong-base titration will be at a slightly basic pH. This is because the base is stronger and dissociates to a greater degree, while the acid is not quite and strong and doesn't dissociate to a large enough extent to neutralize each equivalent of the base.

In this question, titration curve would graph the pH of acid solution versus the amount of base added. Since the base is strong and the acid is weak, we can conclude that the pH will be slightly greater than 7 at the equivalence point. The equivalence point is found in the steepest region of the curve.

The half-equivalence point is the flattest region of the titration curve and is most resistant to changes in pH. This corresponds to the pKa of the acid. Within this region, adding base (changing the x-value) results in very little deviation in the pH (the y-value). This region is also the buffer region for the given acid.

First, let us write out the reaction that occurs:

$HCl_{(aq)}$$+NaOH_{(aq)}$rightarrow $NaCl_{(aq)}$+H_$2O_{(l)}$

32.1hspace{1 mm}mLtimes$\frac{1hspace{1 mm}$L}{1000hspace{1 mm}mL}times$\frac{0.0540hspace{1 mm}$moleshspace{1 mm}NaOH}{1hspace{1 mm}L}times$\frac{1hspace{1 mm}$molehspace{1 mm}HCl}{1hspace{1 mm}molehspace{1 mm}NaOH}=1.73times $10^{-3}$hspace{1 mm}moleshspace{1 mm}HCl

$\frac{1.73times $10^{-3}$$hspace{1 mm}moleshspace{1 mm}HCl}{500hspace{1 mm}mL}times$\frac{1000hspace{1 mm}$mL}{1hspace{1 mm}L}=3.47times $10^{-3}$hspace{1 mm}M

If we are working with a monoprotic acid, our chemical equation is:

NaOH+HXrightarrow NaX+H_2O

Now we will calculate the moles of HX in our solution:

36hspace{1 mm}mLtimes $\frac{1hspace{1 mm}$L}{1000hspace{1 mm}mL}times $\frac{0.4hspace{1 mm}$moleshspace{1 mm}NaOH}{1hspace{1 mm}L}times$\frac{1hspace{1 mm}$moleshspace{1 mm}HX}{1hspace{1 mm}molehspace{1 mm}NaOH}=1.44times $10^{-2}$hspace{1 mm}moleshspace{1 mm}HX

Now we will determine the concentration from the amount of moles and the volume

$\frac{1.44times $10^{-2}$$hspace{1 mm}moleshspace{1 mm}HX}{500hspace{1 mm}mL}times$\frac{1000hspace{1 mm}$mL}{1hspace{1 mm}L}=2.88times $10^{-2}$hspace{1 mm}M

When you titrate a weak base, you want to titrate it with a strong acid. Hydrofluoric acid, citric acid, and stearic acid are all weak acids, and sodium hydroxide is a strong base. The best choice it nitric acid, a strong acid.

There are two things to consider here.

1. Since the solution is originally a strong base, the pH will be originally elevated. As a strong acid is added to the solution, the pH will decrease. As a result, the titration curve will be decreasing as the volume of titrant increases.

2. The titration curve will never be a straight line. Eventually, the strong acid will be much larger in volume than the original base; however, the pH will eventually even out at the pH of the added titrant.

Since we are titrating a strong base with a strong acid, the titration curve will be represented by a decreasing sigmoidal curve.

To find the molecular weight, we must determine the number of moles that correspond to the 0.458g sample. At the equivalence point, the moles of hydronium ions will equal the moles of hydroxide ions.

We need to use the molarity and volume of the NaOH that was added to find the number of moles of base added. This will tell us the moles of hydroxide ions.

Now, since we are working with a diprotic acid, two moles of base would be required for every one mole of the acid. The moles of acid would be:

Now that we know the moles of acid in the sample, we can use the given sample mass to find the molecular weight.

To determine the predicted color change, the pH of the solution must be determined.

The molar mass of is approximate .

is a strong base and dissociate completely in a 1:1 ratio into its counterions ( and ).

The concentration of determines the pH of the solution and may be found by calculating the number of moles of dissolved in solution and dividing by the total volume:

Next, we find the pOH of the solution, from which the pH may be easily determined:

According to the given figure, a pH of 10 corresponds to a color change to blue.

The equivalence point for a strong-acid / strong-base titration will be at neutral pH, 7. This is because each equivalent of the acid will neutralize each equivalent of the base, and you will be left with a neutral solution.

The equivalence point for a strong-acid / weak-base titration will be at a slightly acidic pH. This is because the acid is stronger and dissociates to a greater degree, while the base is not quite as strong, so doesn't dissociate to a large enough extent to neutralize each equivalent of the acid.

The equivalence point for a weak-acid / strong-base titration will be at a slightly basic pH. This is because the base is stronger and dissociates to a greater degree, while the acid is not quite and strong and doesn't dissociate to a large enough extent to neutralize each equivalent of the base.

In this question, titration curve would graph the pH of acid solution versus the amount of base added. Since the base is strong and the acid is weak, we can conclude that the pH will be slightly greater than 7 at the equivalence point. The equivalence point is found in the steepest region of the curve.

The half-equivalence point is the flattest region of the titration curve and is most resistant to changes in pH. This corresponds to the pKa of the acid. Within this region, adding base (changing the x-value) results in very little deviation in the pH (the y-value). This region is also the buffer region for the given acid.

First, let us write out the reaction that occurs:

$HCl_{(aq)}$$+NaOH_{(aq)}$rightarrow $NaCl_{(aq)}$+H_$2O_{(l)}$

32.1hspace{1 mm}mLtimes$\frac{1hspace{1 mm}$L}{1000hspace{1 mm}mL}times$\frac{0.0540hspace{1 mm}$moleshspace{1 mm}NaOH}{1hspace{1 mm}L}times$\frac{1hspace{1 mm}$molehspace{1 mm}HCl}{1hspace{1 mm}molehspace{1 mm}NaOH}=1.73times $10^{-3}$hspace{1 mm}moleshspace{1 mm}HCl

$\frac{1.73times $10^{-3}$$hspace{1 mm}moleshspace{1 mm}HCl}{500hspace{1 mm}mL}times$\frac{1000hspace{1 mm}$mL}{1hspace{1 mm}L}=3.47times $10^{-3}$hspace{1 mm}M

If we are working with a monoprotic acid, our chemical equation is:

NaOH+HXrightarrow NaX+H_2O

Now we will calculate the moles of HX in our solution:

36hspace{1 mm}mLtimes $\frac{1hspace{1 mm}$L}{1000hspace{1 mm}mL}times $\frac{0.4hspace{1 mm}$moleshspace{1 mm}NaOH}{1hspace{1 mm}L}times$\frac{1hspace{1 mm}$moleshspace{1 mm}HX}{1hspace{1 mm}molehspace{1 mm}NaOH}=1.44times $10^{-2}$hspace{1 mm}moleshspace{1 mm}HX

Now we will determine the concentration from the amount of moles and the volume

$\frac{1.44times $10^{-2}$$hspace{1 mm}moleshspace{1 mm}HX}{500hspace{1 mm}mL}times$\frac{1000hspace{1 mm}$mL}{1hspace{1 mm}L}=2.88times $10^{-2}$hspace{1 mm}M

When you titrate a weak base, you want to titrate it with a strong acid. Hydrofluoric acid, citric acid, and stearic acid are all weak acids, and sodium hydroxide is a strong base. The best choice it nitric acid, a strong acid.

There are two things to consider here.

1. Since the solution is originally a strong base, the pH will be originally elevated. As a strong acid is added to the solution, the pH will decrease. As a result, the titration curve will be decreasing as the volume of titrant increases.

2. The titration curve will never be a straight line. Eventually, the strong acid will be much larger in volume than the original base; however, the pH will eventually even out at the pH of the added titrant.

Since we are titrating a strong base with a strong acid, the titration curve will be represented by a decreasing sigmoidal curve.

To find the molecular weight, we must determine the number of moles that correspond to the 0.458g sample. At the equivalence point, the moles of hydronium ions will equal the moles of hydroxide ions.

We need to use the molarity and volume of the NaOH that was added to find the number of moles of base added. This will tell us the moles of hydroxide ions.

Now, since we are working with a diprotic acid, two moles of base would be required for every one mole of the acid. The moles of acid would be:

Now that we know the moles of acid in the sample, we can use the given sample mass to find the molecular weight.

To determine the predicted color change, the pH of the solution must be determined.

The molar mass of is approximate .

is a strong base and dissociate completely in a 1:1 ratio into its counterions ( and ).

The concentration of determines the pH of the solution and may be found by calculating the number of moles of dissolved in solution and dividing by the total volume:

Next, we find the pOH of the solution, from which the pH may be easily determined:

According to the given figure, a pH of 10 corresponds to a color change to blue.