standardization

1. Sodium hydroxide (NaOH) is deliquescent and will readily absob water from the air. It also readily absorbs carbon dioxide from the air. Measurement of mass, i.e. molarity of standard solution so prepared, will thus be inaccurate.
For hydrated Sodium Carbonate (Na2CO3．xH2O), you do not know the exact amount of water of crystallization it contains and thus cannot measure the mass of Na2CO3 simply by weighing.
2. The range of 20 - 25 cm³ should be adopted in order to minimize error due to our reaction time.
In doing titration, there is always a slight reaction time lag after we see the colour change and close the stop cock of the burrette. Thus the actual amount of solution added may be slightly more than that required to reach the end point. If the range of 3-4 cm³ is used, the effect of and thus the error so induced by this additional droplet of solution would be much significant than adopting the range of 20 - 25 cm³.
3. To minimize the chance of reducing the no. of moles of Na2CO3 pipetted, thus consequential errors.
If only water is used to rinse the pipette, remaining droplets of water would reduced the volume and hence no. of moles of Na2CO3 pipetted. This would cause a negative deviation from the correct result when calculating the concentration of the reactant.
4. Yes, as remaining droplets of standard sodium carbonate would increase the actual no. of moles of Na2CO3 in the conical flask, i.e. no. of moles of Na2CO3 participating in the reaction. This would lead to a positive deviation from the correct result when calculating the concentration of the reactant.

1.both sodium hydroxide and sodium carbonate will absorb water vapour in the air which affect the mass measure

2. 3-4is more appropreiate as the same volume and molarity of substance should consume same amount of titrant.The range must be small

3 .If you use Na2CO3 as reagent pipette must be rinsed with it because water inside pipette will reduce the volume of Na2CO3 pipetted ,say 0.1cm3 water inside the 25.0cm3 pipette
only 24.9 Na2CO3 will fill inside the pipette
dilution of the Na2CO3 will cause less mole of reagent present in conical flask,less titrant will be consumed by the fewer reagent to reach the end point.(negative deviation)

4.rinsing conical flask with the reagent must cause more titrant to be used because in addition to the Na2CO3 pipette,some more present in conical flask as you rinse with it
more titrant must be added to reach the end point.(positive deviation)