Why do soluble salts have to be made by titration, using an indicator?

This technique can be used to make pure crystals of a soluble salt.
The burette is filled with hydrochloric acid.
A known quantity of alkali (say 50 cm3 sodium hydroxide)
is released from a pipette into the conical flask.
The tap on the burette is turned open to allow
the acid to be added drop by drop into the alkali.
The alkali contains an indicator (phenolphthalein)
which is pink in an alkali and colorless in an acid.

When enough acid has been added to neutralize 
the alkali, the indicator changes from
pink to colorless. This is the end point of the titration.

The titration can be repeated using the same amounts
of acid and alkali but without the indicator.
Pure salt crystals which are free from indicator
can then be crystallized  from the neutral solution.

From the periodic table:
molar mass of sodium = 23 gm
molar mass of Cr = 51.99 gm
molar mass of oxygen = 16 gm

molar mass of Na2Cr2O7 = 2(23) + 2(51.99) + 7(16) = 261.98 gm

number of moles in 7 gm = mass / molar mass = 7 / 261.98 = 0.0267 moles

1 mole of  Na2Cr2O7  contains 7 moles of oxygen, therefore:
number of moles of oxygen in 0.0267 moles = 0.0267 x 7 = 0.1869 moles

To find the number of atoms, we multiply the number of moles by Avogadro's number as follow:
number of atoms = 0.1869 x 6.02 x 10^23 = 1.125138 x 10^23 atoms