Question

The experiment is Determination of Acetic Acid in Vinegar. Please answer as many questions as you can.

What are primary and secondary standards? Give examples.

What is KHP and why was it used in this lab?

How is acetic acid formed in vinegar solution?

Why are pH indicators strongly colored even in solid state?

Why/how do pH indicators change colors in acid-base solutions?

Molar mass of acetic acid is often calculated or experimentally measured as 120 g/mol. Why?

Suggest another analytical method to determine the end point of an acid-base titration.

pH indicators are often weak acids or bases. Would adding indicators interfere with the determination of the endpoint for desired acid-base titration?

If a solution of NaOH is prepared and standardized today and left on the bench for a week, would you expect the pH of the solution change? Why? Ignore solvent (water) evaporation.

You had titrated unknown vinegar solution in flask against standard NaOH from burette using phenolphthalein indicator. If you had titrated standard NaOH solution in flask against unknown vinegar solution in burette using the same indicator, would it be easier or more difficult? Why?

Answer 1

What are primary and secondary standards? Give examples.

Ans;-

A primary standard is a reagent that is extremely pure, stable, it has no water of hydration, and has a high molecular weight. E.g. sodium carbonate: Na2CO3, mol wt. = 105.99 g/mol, tris- (hydroxymethyl) aminomethane (CH2OH)3CNH2, mol wt. = 121.14 g/mol.

A secondary standard is a standard which is usually standardized against a primary standard. Eg standard solutions of Acids and Bases

What is KHP and why was it used in this lab?

Ans;-

Potassium hydrogen phthalate, KHP, is an acidic salt compound. It is an ionic solid ,mono potassium salt of phthalic acid. KHP used as aprimary standard for acid-base titrations because it is solid, stable and easy to weigh accurately. It is also used as a primary standard for calibrating pH meters. KHP dissociates completely in water,

KHP ⇌ HP ^- + K⁺

HP ^- + H₂O ⇌ P²⁻ + H₃O⁺

How is acetic acid formed in vinegar solution?

Ans:-

when yeasts change natural sugars to alcohol under controlled conditions, the process is called alcoholic fermentation. Further a group of bacteria called Acetobacter converts the alcohol portion to acetic acid. This acetic acid solution is called vinegar which contains about 5–20% acetic acid (CH₃COOH), water, and other trace chemicals.

Why are pH indicators strongly colored even in solid state?

pH indicators are frequently weak acids or weak bases consist of poly aromatic rings containing one or more chromophores ( colour producing groups). The depth of the colour depends up on the numbers of poly aromatic rings/ conjugated system and number of chromophores.    .

Why/how do pH indicators change colors in acid-base solutions?

pH indicators are substances which change colour with pH change, when dissolved in water dissociate slightly and form ions.

HIn (aq) + H2O ⇌ H₃O⁺ + In- The colour of these species like HIn or In- depends on the pH of the solution. As the pH of the solution changes the colour also changes.