Chemistry 9729 P4 Procedures

What is the procedure for Titration?

1) Pipette 25.0cm³ of solution A into a conical flask

2) Add a few drops of #indicator into the flask (if necessary) 

3) Titrate the solution in the flask with solution B in the burette,

• adding dropwise towards the end-point,

• until the colour changes from ____ to _____ at the end point 

4) Repeat the titration until 

• to obtain 2 consistent titres within 0.10cm³ in difference

• Record all the titration results and average the two consistent titres 

What is the procedure of dilution?

1) Using a 50.00cm³ burette, run #V cm³ of solution Y into a volumetric flask

2) Make up to the graduated mark with deionised water

3) Stopper and shake the flask to obtain a homogenous solution 

What is the standard solution preparation procedure?

1. Weigh an empty weighing bottle using a weighing balance which is precise to 3d

• Record its mass, m1

2. Weigh accurately #m g of solid X into the weighing bottle

• Record the total mass of the weighing bottle and solid X, m2

• Mass of solid X used = (m2-m1)

3. Transfer the weighed solid X into a small beaker. Rinse the weighing bottle with some deionised water and transfer the washings into the beaker

• Stir with a glass rod to dissolve solid X completely with deionised water 

4. Using a filter funnel, carefully transfer the solution and all the washings into a 250cm³ graduated flask

5. Stopper and shake the flask to obtain a homogenous solution 

What are type of indicators

Strong Base in burette: phenolphtalein 

• colourless to pale pink 

Strong Acid in burette: methyl orange

• yellow to orange

What is Gravimetric Analysis?

amount of chemical (mol) is determined through measurement of masses

Why hygroscopic substances cannot use GA?

• Readily absorb moisture when exposed to air

• mass of sample measured may not be the actual mass

What is the procedure for heating to constant mass?

1. Weigh a dry, empty boiling tube/ crucible using an electronic balance which is precise to 3dp

• Record its mass

2. Add about #m g of the solid sample into the BT/C and record the total mass

• (of the boiling tube/ crucible and the soild used)

3. Using a bunsen burner, heat the BT/C with its contents gently first for about 1 to 2min

• then strongly for about 5-10min 

4. After heating, place the hot BT/C on a heat proof mat to cool to rtp

• After cooling, weigh and record the total mass of the BT/C after the first heating

5. Repeat the process of heating and cooling and re-weighing of BT/C containing the residue until a constant mass is obtained.

• Record all the masses

Why must the lid be on before it is heated strongly?

Spurting of solid can occur if the crucible is heated too strongly at first

→ due to the sudden expansion of air trapped in the solid sample

How to ensure decomposition is complete in GA?

Stir the solid sample to ensure even heating

Why must cool to rtp before reweighed?

Convectional air current inside the BT/C

→ causing fluctuations in mass readings 

Why must the lid be on while cooling?

To prevent absorption of water by residue

→ If not mass of residue recorded will be higher 

How to improve accuracy of mass of residue?

Cool the crucible with the residue in a desiccator

→ air tight container with drying agent

→ prevent contact with air/ moisture during cooling 

What is the gas collection procedure?

1. Tie a small tube containing the weighed solid sample with a thread and carefully hang the tube inside a conical flask 

2. Using a #suitable apparatus, measure #V cm³ of #aq reagent into the conical flask fitted to a 100cm³ frictionless graduated gas syringe and stopper the flask 

3. Record the initial volume reading on the gas syringe. 

4. Pull the thread to tip the solid into #aq reagent. Shake the flask to mix the reactants well 

5. When the piston stops moving, record the final reading on the graduated gas syringe 

Why is total volume kept constant for kinetics experiments? 

Initial concentration of reactant in reaction mixture is directly proportional to the volume of reactant used 

How to conduct continuous method for kinetics?

One reaction mixture is observed over time.

How to conduct initial rates method for kinetics?

Several different reaction mixtures are prepared

• conc of one reactant is varies

• while keeping conc of all other reactants constant

How do you describe and explain an autocatalysis reaction

At the start:

-Gradient of tangent is gentle/ slightly negative 

• indicates rate of rxn is slow

→ Low [ ion ] 

As reaction proceeds: 

-Gradient of tangent becomes steeper

• indicates rate of rxn increases

→ Due to high [ ion ] 

Towards the end: 

-Gradient of tangent becomes less negative 

• indicates rate of rxn decreases

→ Due to [ ion ] decreases