Titration Error and Back Titration

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Questions and Answers

What is the concentration of Cl- after mixing the solutions?

0.007143 M (correct)

0.02500 M

0.0500 M

0.100 M

What does the Ksp expression for AgCl involve?

Cl- only

Na+ and Cl- concentrations

Ag+ only

Ag+ and Cl- concentrations (correct)

If the Ksp for AgCl is 1.82 x 10^-10, what is the concentration of Ag+ when Cl- is 0.02500 M?

7.0 x 10^-9 M

2.548 x 10^-8 M (correct)

3.64 x 10^-10 M

1.82 x 10^-8 M

What would be the pAg if the concentration of Ag+ is 2.548 x 10^-8 M?

7.59 Signup and view all the answers

What happens to the concentration of NaCl when AgNO3 is added?

It decreases Signup and view all the answers

In the dissolution of NaCl, what ions are produced?

Na+ and Cl- Signup and view all the answers

When calculating the Ksp for AgCl, which ion concentration is held constant?

Cl- Signup and view all the answers

At what volume of AgNO3 does the number of mol NaCl remaining stop decreasing?

30.00 ml Signup and view all the answers

What characterizes the equivalence point in a titration?

Major change in the relative concentrations of reagent and analyte Signup and view all the answers

What type of curve is plotted using p-function as a function of reagent volume?

Sigmoidal curve Signup and view all the answers

What type of calculations are required to construct a titration curve?

Pre-equivalence, equivalence, and post-equivalence Signup and view all the answers

What is the primary reaction type used in precipitation titrimetry?

Reactions yielding ionic compounds of limited solubility Signup and view all the answers

Why is titration with silver nitrate commonly used?

It determines halide ion concentration in aqueous solutions Signup and view all the answers

What term is associated with titrimetric methods based on silver nitrate?

Argentometric methods Signup and view all the answers

Which of the following describes a linear segment curve in titration?

Measurements are made on both sides of the equivalence point but away from it Signup and view all the answers

What is the significance of Ksp in a precipitation reaction?

It indicates the solubility product constant related to the precipitate Signup and view all the answers

What is the formula for calculating titration error?

Et = Vep - Veq Signup and view all the answers

In a back titration, what does the equivalence point correspond to?

The volume of analyte and the back titrant combined Signup and view all the answers

Which of the following is an example of a primary standard?

Potassium hydrogen phthalate (KHP) Signup and view all the answers

What is the purpose of back titration?

To quantify an analyte that does not react directly with the titrant Signup and view all the answers

Which reaction accurately represents a step in the back titration of phosphate using silver nitrate?

3Ag+ + PO4^3- → Ag3PO4(s) Signup and view all the answers

Which property is essential for a compound to be considered a primary standard?

It must be highly purified and stable Signup and view all the answers

If the actual volume of reagent needed for titration is 25 mL and the theoretical volume to reach the equivalence point is 20 mL, what is the titration error?

5 mL Signup and view all the answers

Which statement is true regarding the accuracy of titration methods?

It relies significantly on the properties of primary standards. Signup and view all the answers

What is the calculated concentration of Ag+ at the equivalence point when 50.00 mL of 0.0500 M NaCl is titrated with 0.100 M AgNO3?

0.00625 M Signup and view all the answers

Which statement accurately represents the relationship between concentration and pAg values in titration curves?

Dilute solutions exhibit smaller changes in pAg values. Signup and view all the answers

How does the solubility product constant (ksp) impact the change of pAg at the equivalence point?

Smaller ksp results in greater pAg changes at equivalence. Signup and view all the answers

What is the value of ksp for the reaction involving Ag+ and Cl- ions?

1.82 x 10-10 Signup and view all the answers

Which of the following concentrations was used to titrate 50.00 mL of 0.00500 M NaCl?

0.0100 M AgNO3 Signup and view all the answers

What can be said about the relationship of [Ag+] and [Cl-] at the equivalence point?

[Ag+] = [Cl-] at the equivalence point. Signup and view all the answers

What happens to the pAg of a solution as the concentration of the reactants decreases?

pAg changes less significantly with lower concentrations. Signup and view all the answers

What would be the pAg calculated from a concentration of 1.82 x 10-5 M Ag+?

4.87 Signup and view all the answers

What type of end point is determined by measuring the potential difference between a silver electrode and a reference electrode?

Potentiometric end point Signup and view all the answers

Which type of indicator is used in argentometric titrations that results in a color change during the titration?

Chemical indicator Signup and view all the answers

What is true about Ksp values in relation to precipitation curves?

Smaller Ksp values give much sharper breaks at the end point. Signup and view all the answers

What does an amperometric end point involve during the titration process?

Monitoring the current generated by silver microelectrodes. Signup and view all the answers

In an argentometric titration, which of the following solutions is likely to be titrated with AgNO3?

A solution containing anions Signup and view all the answers

Which condition is NOT a requirement for an indicator used in precipitation reactions?

The color change should occur over a broad range of pH. Signup and view all the answers

What measurement is plotted during an amperometric titration?

Current vs. volume of added reagent Signup and view all the answers

What characteristic of titration curves is similar for potentiometric and other types of titrations?

They show sharp inflection points indicating end points. Signup and view all the answers

Study Notes

Titration Error

Titration error is the difference between the equivalence point and the end point.
The formula for calculating titration error is Et = Vep - Veq.
Vep is the actual volume of reagent needed to reach the end point, and Veq is the theoretical volume needed to reach the equivalence point.

Back Titration

Back titration is a process that is sometimes necessary when an excess of standard titrant is added.
The excess of standard titrant is then determined by back titration with a second standard titrant.
An example of back titration is determining the amount of phosphate (PO43-) in a sample.
A measured excess of standard silver nitrate is added to a solution of the sample.
This leads to the formation of silver phosphate (Ag3PO4(s)) and the excess silver nitrate is back-titrated with a standard solution of potassium thiocyanate (Ag+ + SCN- → AgSCN(s)).
The amount of silver nitrate is chemically equivalent to the amount of phosphate ion plus the amount of thiocyanate used for the back titration.

Primary Standards

A primary standard is a highly purified compound that serves as a reference material in all volumetric and mass titrimetric properties.
A few examples of primary standards are sodium carbonate (Na2CO3)and potassium hydrogen phthalate (KHP).
The accuracy of a titrimetric method is dependent upon the properties of the compound.

Types of Titration Curves

Titration curves are plots of a concentration-related variable, such as a p-function, as a function of reagent volume.
Titration curves are used to understand the theoretical basis of the end point and sources of titration errors.
There are two main types of titration curves:
- Sigmoidal Curve: The p-function of the analyte or reagent is plotted against the reagent volume.
- Linear Segment Curve: Measurements are made on both sides, but well away from, the equivalence point.

Precipitation Titrimetry

Precipitation titrimetry is based on reactions that yield ionic compounds of limited solubility.
It is one of the oldest analytical techniques.
Titrimetric methods based on silver nitrate are sometimes called argentometric methods.
Argentometric is derived from the Latin word argentum, meaning "silver."

Precipitation Titration Curves Involving Silver Ions

The most common method of determining the halide ion concentration of an aqueous solution is titration with a standard solution of silver nitrate.
To construct a titration curve, three types of calculations, corresponding to distinct stages in the reaction, are required:
- Pre-Equivalence (Before): The concentration of analyte is calculated before the equivalence point is reached.
- Equivalence (At): The concentration of analyte is calculated at the equivalence point.
- Post-Equivalence (After): The concentration of the excess titrant is calculated after the equivalence point is reached.
The titration curves for this method are the plot of pAg versus the volume of AgNO3.

The Effect of Concentration on Titration Curves

The lower the concentration, the smaller the pAg values become.
For more dilute solutions, the change in pAg is small.

The Effect of Reaction Completeness on Titration Curves

The change of pAg at the equivalence point becomes greater as the solubility products become smaller.
This means the reaction between analyte and AgNO3 becomes more complete.
For each curve, 50.00 mL of a 0.0500M solution of the anion was titrated with 0.1000 M AgNO3.
Smaller values of Ksp give much sharper breaks at the end point.

Titration Curves for Mixtures of Anions

The titration curves for 50.00mL of a solution 0.0800 M in Cl- and 0.0500 M in I- or Br- are shown below.

Indicators for Argentometric Titrations

There are three types of endpoints involved in silver nitrate titrations:
- Potentiometric Endpoint: This is obtained by measuring the potential between a silver electrode and a reference electrode.
- Amperometric Endpoint: This is obtained by measuring current generated by a pair of silver microelectrodes as a function of the reagent volume.
- Chemical Endpoint: This is produced by chemical indicators, which cause a change in color during titration.

Potentiometric Endpoint

The titration curves for potentiometric titrations are similar to those discussed in previous sections.
The potential difference between a silver electrode and a reference electrode is measured to determine the equivalence point.

Amperometric Endpoint

The instrument used for amperometric endpoint detection consists of a pair of silver microelectrodes immersed in the analyte solution.
The current generated by these electrodes, as a function of the reagent volume, is measured and plotted to determine the equivalence point.

Chemical Endpoint

Chemical endpoints are produced by chemical indicators that cause a color change during titration.
The requirements for an indicator for precipitation are:
- The color change should occur over a limited range in the p-function for either the reagent or the analyte.
- The indicator should be highly soluble.

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Description

Explore the concepts of titration error and back titration in this quiz. Understand the difference between the equivalence point and the endpoint, as well as how to calculate titration error. Test your knowledge with examples involving standard titrants and phosphate determination.