ib chem paper 3

Explain the increase in molecular dipole moment as one chlorine atom in CCl4 is replaced with fluorine to produce CCl3F. (2)

C–F bond more polar «than C–Cl bond» / F more e neg

the dipoles of the three » C–Cl bonds do not cancel the dipole of the C–F bond

A student wished to determine the concentration of a solution of sodium hydroxide by titrating it against a 0.100moldm−3 aqueous solution of hydrochloric acid. 4.00g of sodium hydroxide pellets were used to make 1.00dm3 aqueous solution.

(a) Outline, giving your reasons, how you would carefully prepare the 1.00dm3 aqueous solution from the 4.00g sodium hydroxide pellets (3)

• use volumetric flask and mix the solution

• mix pellets in beaker with distilled water

• safety: let the solution cool because NaOH reacts with water exothermically

The student added the acid too quickly. Outline, giving your reason, how this could have affected the calculated concentration. (2)

• equivalence point been exceeded

• thus the concentration increases

Suggest why, despite preparing the solution and performing the titrations very carefully, widely different results were obtained

• usually impurities work unless the question says its pure

• temperature changes during experiment

• incorrect method of calculation