Unit 16: Lab Safety and Practical Skills — Short Questions

Methyl orange is used for strong acid-weak base titrations because it changes color in the acidic pH range (around pH 3.1-4.4), making it suitable for detecting the acidic end point.

In a weak acid-strong base titration, the equivalence point lies in the basic pH range where phenolphthalein changes color (pH 8.3-10). However, in a strong acid-weak base titration, the equivalence point is acidic, so phenolphthalein would remain colorless and is unsuitable.

Each indicator is a weak acid or base and has a unique pKa value. The color change occurs over a specific pH range where the ratio of the protonated and deprotonated forms changes, leading to visible color changes.

Dilute solutions help to avoid sudden or violent reactions, ensure more accurate titration results, and make color changes easier to detect near the end point.

• Ca²⁺: Precipitate of Ca(OH)₂ is insoluble in excess NaOH.
• Al³⁺: Precipitate of Al(OH)₃ dissolves in excess NaOH forming a colorless solution.
• Zn²⁺: Zn(OH)₂ also dissolves in excess NaOH but can be confirmed with H₂S, forming a white precipitate of ZnS.

• Add potassium ferricyanide: Fe²⁺ gives a blue precipitate (Turnbull's blue).
• Add potassium thiocyanate: Fe³⁺ gives a deep red solution due to the formation of Fe(SCN)₃.

Aqueous ammonia does not produce a sufficiently high OH⁻ concentration to precipitate Ca(OH)₂ due to its low solubility product (Ksp); hence, no visible precipitate forms.

By performing an acid-base titration using NaOH of known concentration as the titrant and phenolphthalein as the indicator. The volume of NaOH used to reach the end point is used in the molarity equation to calculate the acetic acid concentration.

Always add acid to water slowly, never the reverse, while stirring continuously. This prevents violent splashing due to the exothermic nature of the dilution.

The carbonate ion (CO₃²⁻) hydrolyzes in water to form OH⁻ ions, making the solution basic.

NaOH absorbs carbon dioxide (CO₂) from air, forming sodium carbonate (Na₂CO₃), thus reducing the concentration of hydroxide ions in the solution over time.

Titration is a volumetric analysis technique used to determine the concentration of a solution by reacting it with a standard solution of known concentration until the reaction reaches completion.

An indicator is a substance that shows the completion of a reaction by changing its color.

Phenolphthalein.

To prevent any extra drops of the solution from entering the burette, which could affect the accuracy of the initial reading.

To clearly observe the color change of the indicator.

HCl solution is taken in the burette and NaOH solution in the conical flask.

• Use clean and rinsed apparatus with the appropriate solution.
• Ensure no air bubbles are in the burette nozzle.
• Remove the funnel before starting the titration.
• Add the titrant (acid) slowly near the endpoint.
• Swirl the flask continuously for proper mixing.
• Read the burette at eye level using an anti-parallax card.
• Repeat the titration to get concordant readings.

A chemistry laboratory is a chemist's workspace where students learn to observe physical and chemical properties of substances by following definite procedures.

Add 4g sample in a clean test tube. Dissolve it in 10 cm³ distilled water. Make two parts of the above solution for further tests.

To one part, add NaOH solution.

• Observation: A green precipitate appears which turns into green solution when excess of NaOH is added.
• Inference: Cr³⁺ ions are indicated.

To the second part of the above solution, add aqueous ammonia.

• Observation: A green precipitate appears which is insoluble in excess of aqueous ammonia.
• Inference: Cr³⁺ ions are indicated.

Cr³⁻(aq) + 3OH⁻(aq) → Cr(OH)₃(s)
Green ppt

Take 4g of sample in a clean test tube. Dissolve it in 10cm³ distilled water. Add NaOH solution and gently heat it.

• Observation: Ammonia gas is evolved with a distinct smell. Ammonia gas turns moist red litmus blue.
• Inference: NH₄⁺ ions are indicated.

NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

Take 4g solid sample in a clean test tube. Dissolve it in 10cm³ distilled water. Make two parts of this solution for further tests.

To one part, add NaOH solution.

• Observation: White precipitate is formed which does not dissolve in excess of NaOH solution.
• Inference: Ca²⁺ ions are indicated.

To the second part of the above solution, add aqueous ammonia.

• Observation: Only a slight turbidity appears or No precipitate is formed.
• Inference: Ca²⁺ ions are indicated.

Ca²⁺(aq) + 2OH⁻(aq) → Ca(OH)₂(s)
white ppt

See Q.2 from theory.

See Q.4 from theory.

See Q.6 from theory.

See Q.7 from theory.