unit 4 chemistry

What is colorimetry used for?

To determine the concentration of colored substances in a solution by measuring light absorbance

What are the main components of a colorimeter?

1. Light source

2. Colored filter

3. Glass cuvette

4. Light detector and recorder.

What else does distillation do apart from purify

Identify

What is a calibration graph used for?

To determine the concentration of an unknown solution by comparing its absorbance to the graph of absorbance vs. concentration

How is a calibration graph prepared?

1. Make 5-7 standard solutions with known concentrations.

2. Measure absorbance of each solution using a colorimeter.

3. Plot absorbance vs. concentration.

What is a standard solution?

A solution with a known and precise concentration.

How is a standard solution prepared from a primary standard?

1. Weigh the primary standard accurately.

2. Dissolve in a small volume of distilled water.

3. Transfer to a volumetric flask and add rinsings.

4. Make up to the mark with water.

5. Stopper and mix by inverting.

What are the properties of a primary standard?

1. High purity

2. Stability in solid and solution

3. Solubility in water

4. High formula mass.

Give examples of primary standards.

Sodium carbonate, potassium dichromate, silver nitrate

Why can't sodium hydroxide be used as a primary standard?

Sodium hydroxide absorbs CO2 and moisture from the air, making it unstable. It also cannot provide precise concentration

How can you prepare a standard solution by dilution?

1. Pipette a known volume of an existing standard solution into a volumetric flask.

2. Make up to the mark with distilled water.

3. Stopper and invert to mix.

Describe the process of determining manganese in steel.

1. Dissolve the steel sample in nitric acid to form Mn2+

2. Oxidize Mn2+ to MnO - 4 using potassium periodate.

3. Dilute to volume in a volumetric flask.

4. Measure absorbance of MnO-4

5. Use the calibration graph to find concentration and calculate manganese content.

What is dilution?

Dilution is when you add more solvent to a solution, decreasing its concentration, but the amount of solute stays the same.

What formula is used for dilution calculations?

C1V1=C2V2

• C_1 = initial concentration

• V_1 = initial volume

• C_2 = final concentration

• V_2 = final volume

What is ppm?

Ppm means "parts per million" and represents milligrams of solute per kilogram of solvent.

A 1500 g steel plate contains manganese at 62 ppm. What is the total mass of manganese?

62 x 1.5 =93 mg

What are the steps of lab synthesis?

Preparation (refluxing)

Isolation (distillation, vacuum filtration and solvent extraction)

Purification (recrystallisation)

Identification (TLC and MP)

Percentage Yield

Why reflux?

Allows a reaction to be heated for a long time without losing products or reactants.

How do you set up a reflux apparatus?

1. Use a round-bottomed flask with anti-bumping granules.

2. Attach a vertical condenser with water inlets/outlets.

3. Heat gently using a water bath or hot plate.

What process separates liquids based on boiling points

Distillation

How is a distillation setup arranged?

1. A distillation flask with a thermometer in the neck.

2. A horizontal condenser attached to a collection flask.

3. Heat gently and collect the desired fraction.

What is vacuum filtration used for?

To quickly separate a solid from a liquid.

How do you perform vacuum filtration?

1. Place filter paper in a Buchner funnel.

2. Attach the funnel to a vacuum flask connected to a vacuum source.

3. Pour the mixture, then wash and dry the solid.

What method separate a product dissolved in water using an immiscible organic solvent.

Solvent extraction

How do you use a separating funnel for solvent extraction?

1. Add the mixture and organic solvent to the funnel.

2. Shake and let the layers separate.

3. Drain the lower aqueous layer, leaving the organic layer.

A purification method for solids by dissolving in hot solvent and cooling to form crystals.

Recrystallisation

After dissolving the impure solid in a minimum amount of hot solvent what's next in recrystallisation ?

2. Filter to remove insoluble impurities.

3. Cool slowly to form crystals, then filter and dry.

How is melting point used in identification?

To check the purity and identity of a product by measuring its melting range.

1. Place the product in a capillary tube.

2. Insert the tube into a melting point apparatus.

3. Heat slowly and record the melting range.what are these steps for?

performing melting point analysis

What is TLC (Thin-Layer Chromatography) used for?

To check purity and follow the reaction's progress.

How do you perform TLC?

1. Draw a pencil line near the bottom of the TLC plate.

2. Spot the product solution on the line.

3. Place the plate in a jar with solvent.

4. Let the solvent rise, then observe the spots.

What is the formula for percentage yield?

% yield = actual over theory x 100

Why are anti-bumping granules used during heating?

To prevent violent boiling and ensure smooth bubbling.

What happens if anti-bumping granules are not added during refluxing?

Without anti-bumping granules, the reaction mixture could boil violently, potentially causing splashing and unsafe conditions.

How does vacuum filtration differ from normal filtration?

Vacuum filtration uses reduced pressure to speed up the filtration process, making it ideal for filtering precipitates in a quick and efficient manner.

How does melting point analysis help identify a substance?

The melting point of a substance can be compared to its known literature value. A lower or broader range suggests the presence of impurities.

What is Thin Layer Chromatography (TLC) and how does it assess purity?

A pure substance will show as a single spot, while an impure substance will show multiple spots.

In Thin Layer Chromatography (TLC), if a pure substance is present, what would you expect to see on the plate?

One single spot

In TLC, a pure substance will show as a single spot. The question was specific about a "pure" substance, and in that case, there would only be one spot.

Refluxing ?

Heating a reaction mixture with a condenser to prevent volatile substances from escaping.

• Keeps temperature constant during reactions.

What is recrystallisation used for?

To purify solids by removing impurities.

• Relies on the compound dissolving differently in a solvent at hot and cold temperatures.

How do you choose the right solvent?

The compound should dissolve in the solvent when hot but not when cold.

• The solvent should not react with the compound or impurities.

Why is slow cooling important in recrystallisation?

Slow cooling helps form pure crystals.

• Fast cooling can trap impurities in the crystals.

What is the purpose of melting point analysis?

To check the purity of a compound.

• Pure compounds have sharp and specific melting points.

• Impurities lower the melting point and widen the melting range.

How is the melting point range determined?

The starting temperature is when the solid begins to melt.

• The ending temperature is when it's fully melted.

• Compare the range to the known value of the pure compound.

What is a mixed melting point test?

Mix the product with a pure compound.

• If the melting point doesn't change, they are the same.

• A lower melting point indicates impurities or a different compound.

Why are controls important in experiments?

They help confirm the method is valid.

• Example: Test the mass of pure aspirin to ensure accuracy in your experiment.

What indicates that a compound is pure?

The melting point matches the known value and has a sharp range.

• If the melting point is lower or broader, the compound is impure.

What type of impurities are removed during hot filtration in recrystallisation?

Hot filtration removes insoluble impurities from the solution.

How is the purity of a compound confirmed after recrystallisation?

Purity is confirmed by comparing the melting point range of the product to the literature value.

What happens if a compound is soluble in the solvent at both hot and cold temperatures?

No crystals will form because the compound stays dissolved in the solvent.