Chemistry Reference
In-Depth Information
●
Titration
On p. 124 you saw that it was possible to prepare a
soluble salt by reacting an acid with a soluble base
(alkali). The method used was that of
titration
.
Titration can also be used to fi nd the concentration
of the alkali used. In the laboratory, the titration of
hydrochloric acid with sodium hydroxide is carried
out in the following way.
1
25 cm
3
of sodium hydroxide solution is pipetted
into a conical fl ask to which a few drops of
phenolphthalein indicator have been added
(Figure 8.23). Phenolphthalein is pink in alkaline
conditions but colourless in acid.
Example
Calculate the percentage by mass of water in the
salt hydrate MgSO
4
.7H
2
O. (
A
r
: H = 1; O = 16;
Mg = 24; S = 32)
M
r
for MgSO
4
.7H
2
O
= 24 + 32 + (4 × 16) + (7 × 18)
= 246
The mass of water as a fraction of the total mass of
hydrate
=
126
246
The percentage of water present
=
126
246
× 100
= 51.2%
Questions
1
Calculate the percentage by mass of water in the
following salt hydrates:
a
CuSO
4
.5H
2
O
b
Na
2
CO
3
.10H
2
O
c
Na
2
S
2
O
3
.5H
2
O.
(
A
r
: H
=
1; O
=
16; Na
=
23; S
=
32; Cu
=
63.5)
2
Devise an experiment to determine the percentage of
water of crystallisation present in a salt hydrate of your
choice.
●
Solubility of salts
in water
Water is a very good solvent and will dissolve a
whole range of solutes, including sodium chloride
and copper(
ii
) sulfate, as well as other substances
such as sugar. You can dissolve more sugar than
sodium chloride in 100 cm
3
of water at the same
temperature. The sugar is said to be more
soluble
than the sodium chloride at the same temperature.
We say that the sugar has a greater
solubility
than
the sodium chloride. The solubility of a solute
in water at a given temperature is the number of
grams of that solute which can be dissolved in 100 g
of water to produce a saturated solution at that
temperature.
Figure 8.23
Exactly 25.0 cm
3
of sodium hydroxide solution is pipetted
into a conical fl ask.
