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#### Acid-Base Titrations: Calculate The Concentration Of An Unknown Strong Acid

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### Acid-base titrations

The property of a strong is to dissociate or ionize completely in all aqueous solution and form hydronium ions as a result. Example of dissociation of acid with aqueous solution is

HCl + H2O --------> H3O+ + Cl-

The property is a strong acid is not to dissociate completely with the aqueous solution and does not form hydronium ions. Example of a weak acid is shown below

CH3COOH + H2O  CH3COO- + H3O+

Examples of strong acid are hydrochloric acids (HCl), sulphuric acid (H2SO4), and nitric acid (HNO3). There are also strong bases and example of strong bases are NaOH (Sodium Hydroxide), KOH (Potassium hydroxide), LiOH (Lithium hydroxide) and many more.

Examples of weak acid are HF (Hydrofluoric acid) and, oxalic acid (COOH)2, and CH3COOH (acetic acid). Example of weak base is NH4OH (ammonium hydroxide) and NH3 (Ammonia).

The weak acids and the weal bases are always available as conjugate pairs of acids and bases in any aqueous solution. An equation of such acid base solution is shown below:

HA + H2O  H3O+ + A-

(Acid solution) (Conjugate base)

In the above equation, HA is an acid solution and A- is known as the conjugate base.

The equation of base with conjugate acid is shown below:

A- + H2O  OH- + HA

(Base solution) (Conjugate base)

In the equation above, A- is the base and HA is conjugate acid.

There is a thumb rule which states that weak acids have conjugate bases and weak bases have conjugate acids. From the above stated reactions it can be seen that HA is weak acid A- is the conjugate base.

Titration is mainly considered as a technique that determines concentration level of any unknown solution. In titration, a solution of known concentration is mainly used for determining the concentration of any unknown solution. The known concentration solution is known as titrant and unknown concentration solution is titrand or analyte. In a titration, titrate is added in the analyte through burette in known volume till the reaction gets complete. With the known amount of titrant added in the solution helps in determining the level of concentration in the unknown solution. The indicator is mainly used for signalling the end of reaction. Both the known concentrated solution and the unknown concentrated solution forms an acid and a base pair. These acid base solutions are mainly monitored by their change of pH in the titration progresses.

The indicator that is used in the titration process for signalling is usually added to weak acid or a weak base that is added to the analyte solution. By adding the indicator, the colour of the solution changes with the equivalent point. The actual point at which the indicator changes the colour is called endpoint of the solution. The addition of the indicator in the process of titration helps to spot visually the equivalence point in any acid base titration. The endpoint in a titration mainly refers to that point that indicates the change of colour while conducting the acid base solution.

### Concentration of an unknown strong acid given the amount of base necessary to titrate it

A titration is a technique that is mainly used for working out all the concentration of unknown solution that is based on the chemical reaction with a solution of a known concentration. The process of titration mainly includes adding all the known solution to a known quantity of an unknown solution till the reaction gets complete. For calculating the concentration of the unknown solution that is the analyte, the volume of the titrant is to be measured in the reaction.

For preparing the concentration, the analyte is to be placed in Erlenmeyer flask. Erlenmeyer flask is a conical round bottom flask that has a narrow neck. The titrant is placed in the burette. A burette is a graduated tube glass that has a tap on the end of the tube glass.

For mixing the concentration, the titrant is added to analyte till the end point is reached. The end point is the titration point when the colour of the solution changes. And the end point is indicated by the change of colour. This can be done by adding some drops of phenolphthalein which is a common acid base indicator. The colour that changes in with this reaction changes from pink alkali to some colourless acid.

For calculating the molarity, titration formula is to be used. If the titrant and the analyte in the solution have 1:1 ration of mole, the formula states the molarity of that acid * the total volume of the acid is equal to molarity of the base * the total volume of the base. The molarity is defined as the concentration of the total solution that is expressed as total number of solute moles in per litre of the solution.

If the titrant analyte ratio is not taken as 1 : 1,the version of the formula is modified. An example of the formula that can be used if the ration is not 1: 1 is explained below. If there is 25 ml of NaOH (Sodium Chloride) is taken, then 35 ml of the HCl (hydrochloric acid) of molarity 1.25 M is needed to reach to equivalence point. The concentration of NaOH using the formula with ration 1:1 can be considered as the formula because the hydrochloric acid and the sodium hydroxide have mole ration as 1:1 that one mole of hydrochloric acid actually reacts with only one mole of sodium hydroxide.

The molarity of the acid that is taken in the solution is multiplied by total volume of the acid taken that is 1.25 * 35. The answer of total molarity is 43.75 and the result is then divided by the total volume of base that is 25 ml. the answer for the above taken example is 1.75 M is the total molarity of the base.

### Distinguishing a weak acid-strong base titration from other types of titrations

Titrations are the reactions that takes between selected reactants that is a strong base or a weak base is taken for the titration reaction. There is a titration curve that reflects the total strength of the acid and the base that is taken in to solution. The titration curve shows the change in pH level when the titration is taking place. This titration curve mainly demonstrates the change in the pH of the solution when titration of a strong base is done with a weak acid at the beginning of the solution. The level pf pH changes slowly as well as changes gradually. By this reaction, the buffer system is being formed when titration reaches the equivalence point.

Al the equivalence point and after the equivalence point, the titration curve shown below shows the example that reflects the strength of the acid that is taken for titration and their corresponding base that shows the change of pH in the titration phase. Example that can be taken is NaOH and acid HCl. When the level of NaOH is higher, there is a pH change at the same time and is same as any of the system that is dominated by the NaOH.

Figure 1: Titration curve of weak acid and a strong base

The pH at the initial level of the solution in beginning of titration is considered approx. as that of the pH of the weak acid in water. At some equivalence point, all weak acid can be neutralized as well as converted to the corresponding conjugate base. The conjugate base can be stated as the total number of moles of H+ = the total added number of moles present in OH-. The pH at some equivalence point is not equal to 7. This is because the total production of the conjugate base that is present during the titration process. The solution that results after titration is slightly basic. The endpoint of the titration is that particular point when the colour of the solution changes. The endpoint and equivalence point that is considered in the titration process is not exactly same. The equivalence point is mainly determined by stoichiometry of that particular reaction and the end point of the reaction is the change of colour that comes from the indicator.

Stoichiometry is the study and the calculation of the quantitative relationship which is measurable of all the reactants as well as the products in the chemical reaction that is being taking place. In the titration process, one reagent is weak acid and the other reagent strong base, the curve in the titration process is irregular and the pH level of the acid shifts less by adding small additions of the titrant that is near the equivalence point. This weak acid and string base titration mainly depends on neutralization when the solution is mixed. A buffer solution is mainly used to stabilize the liquid of the pH.

### Strong acid-weak base titrations can be performed with either serving as the titrant

Example of titration of a strong acid with a weak base which is shown below consists of ammonia which is a weak base and a hydrochloric acid which is a strong acid in aqueous phase.

NH3 (aq) + HCl (aq) -> NH4+ (aq) + Cl- (aq)

The total acid that is typically titrated in a base solution. Small amount of acid solution that is added in the titration is known concentrated acid solution and is place in burette. The unknown solution is known as titrant solution. The total known volume of the base with some known concentration is actually placed in the Erlenmeyer flask that is the analyte. The pH measurement that is to be done in titration is done through electrode which is a graph of pH with the volume of titrant that can be made and is shown in the titration curve.

In this particular case of titration, acid is taken in the base solution that is taken for strong acid solution and a weak base solution. In the titration, the pH of strong acid with the weak base is will start at high as well as drop rapidly with addition of the acid. When the equivalence point is reached, the pH will change more rapidly until when that one final drop of the solution will help to cause a rapid change in the pH transition with equivalence point. If the chemical indicator is mainly used for the titration reaction, includes methyl orange which is a good choice of indicator as because it changes from a basic colour to its acidic colour.

In case of a strong acid and a weak base titration, the level of pH ay some equivalence point is not calculated as 7. The equivalence point is below 7. This happens because of the total production for conjugating the acid during the process of titration and has to react with the water for producing such hydronium ions (H3O+).

The example below shows the titration of HCl using the ammonia solution. In the reaction, the conjugate acid that is formed is the NH4+.

NH4+  + H2O -> H3O+ + NH3

The titration is mainly performed as some acid in to base. The following titration having the pH meter in real time helps to generate a curve that is shows equivalence point. The approximate pH of all the equivalence point is known as colorimetric indicator that are mainly used for titration. The equivalence point is mainly a chemical reaction that have equivalent chemical quantities or amount of acid solution or a base solution that are to be mixed for carrying out titration method.

### General shape of a pH vs equivalents graph generated by titrating a polyprotic acid.

A diprotic acid mainly contains two protons (H+) and helps to produce two ions of hydrogen in the aqueous solution. There are certain types of polyprotic acids that have specific names including diprotic acid which has two potential protons that can be donated and a triprotic acid that includes three potential protons that can be donated. The loss of all the sequential ions of hydrogen is rising in less favourable way, there are conjugate bases that are present in the solution.

The monoprotic acid is one acid that are capable to donate a hydrogen ion in each module in the process of dissociation. Polybasic acid mainly contains of two or more than two replaceable atoms of hydrogen. Polyprotic acid can donate or can accept more than a proton.

The monoprotic acids are those acids that are capable of donating a proton in each molecule in the process of dissociation. This process is sometimes called as ionization and is denoted by the symbol HA. The below equation shows the monoprotic acids reaction.

HA (aq) + H2O (I)  H3O+ + A (aq)

The common examples that are included in the monoprotic acids in the minerals acids mainly includes hydrochloric acid (HCl) as well as HNO3 (nitric acid). For all the organic acids, there is presence of group of carboxylic acid and these acids are commonly known as the monocarboxylic acid.

The polyprotic acids are mainly able to donate one proton or more than a proton in per molecule of acid. This polyprotic acid is different from monoprotic acids that are capable of donating only one proton in a molecule. There are certain types of polyprotic acid, there are certain names that includes diprotic acids including two potential protons which can be donated, and also includes triprotic acids that includes three potential protons which can be donated. Example of polyprotic acid is oxalic acid, which is also known as ethanedioic acid. This is a diprotic acid that have the capability to donate two protons.

If there is a dilute solution of an oxalic acid is included in the titration method with solution of sodium hydroxide, the protons in the solution will react and the reaction is provided below as per the neutralization reaction.

COOH                              COONa

| + NaOH (aq) -------->      | + H2O (I)

COOH (aq) COONa (aq)

COONa                            COONa

| + NaOH (aq) -------->      | + H2O (I)

COOH (aq)                     COONa (aq)

The above reaction shows the neutralization of the diprotic acid that includes the oxalic acid. If pH of the titration reaction stated above are recorded and the points are plotted in the volume of the solution of NaOH that were added, there comes a stepwise process of the neutralization reaction. This includes a distinct equivalent point that are to be included in the titration curve.

Figure 2: Titration curve of diprotic acid

The oxalic acid is considered as an example of a particular acid that are able to enter in the reaction that have two available protons, including different Ka values for the purpose of dissociation in each proton. In triprotic system, this can be envisioned. Each reaction helps to proceed with unique Ka value.

### Procedure to identify which, of a given series, would be the best acid-base indicator for a given titration

Many methods can be considered that helps to determine the level pH in a particular solution and helps to determine the equivalence point when the acids and the bases are mixed together. These particular methods mainly ranges by the use of litmus paper, the indicator papers, specifically designed electrodes as well as includes the use of coloured molecules that are included in the solution. The electrodes that are used includes all methods that are visual and depends on the fundamental changes that mainly occurs in the molecule when the pH of the environment basically changes. The molecule which is included helps to change the colour with the level pf pH in the environment that can be used as an indicator.

In the particular equation mentioned below, the weak acid HIn is in the equilibrium states having an ionized anion In-. In the reaction, adding of the acids helps to shift all the indicator equilibrium that are to the left of the anion. Adding the base shifts in the reaction, the indicator equilibrium shifts to right. In case of methyl orange indicator, the HIn is mainly coloured red and ionized In- is in the yellow form.

HIn  H+ + In-

The pH indicators that are included in the titration reaction are employed in the analytical chemistry as well as in biology for determining extent of chemical reaction. The pH indicators are mainly susceptible to make the readings more precise.  For the application that requires accurate measurement includes pH and the pH meter is used frequently in the titration method. Sometimes, blending of different indicators are used for achieving smooth colour that are used in wide range of pH values. All these commercial indicators are mainly used for a rough knowledge for the pH that is necessary. The indicators mainly helps to exhibit the intermediate colours at some pH values that are present in the transition range. The approximate pH of all the equivalence point is known as colorimetric indicator that are mainly used for titration. The solution that results after titration is slightly basic. The endpoint of the titration is that particular point when the colour of the solution changes. The endpoint and equivalence point that is considered in the titration process is not exactly same. The equivalence point is mainly determined by stoichiometry of that particular reaction and the end point of the reaction is the change of colour that comes from the indicator.

In titration, a solution of known concentration is mainly used for determining the concentration of any unknown solution. The indicator that is used in compound of halochromic chemical is added in small amount in the solution. This is because the pH of that particular solution are easily determined for the titration that is being taken place. The approximate pH of all the equivalence point is known as colorimetric indicator that are mainly used for titration. The equivalence point is mainly a chemical reaction that have equivalent chemical quantities or amount of acid solution or a base solution that are to be mixed for carrying out titration method.

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