Acids and Bases Revision Structured Question

Sodium phosphate, Na3PO4, is a soluble salt, used as a water softener in washing
powders. It is made by reacting dilute phosphoric acid, H3PO4, with an alkali.
(a) (i) Give the formulae of the ions present in sodium phosphate.

 

(ii) Name the alkali which reacts with phosphoric acid to make sodium
phosphate.

 

(iii) Write a balanced chemical equation for this reaction.

 

(iv) Given solutions of phosphoric acid and alkali, a suitable indicator and
standard laboratory apparatus, explain how you would obtain crystals of
sodium phosphate.

 

 

 

 

Copper occurs naturally in the ore malachite CuCO3.Cu(OH)2. When malachite
is heated it produces copper(II) oxide, carbon dioxide and water.
Malachite can be readily converted into copper by heating it with carbon.
(b) Write a balanced chemical equation for the action of heat on malachite.

 




Answer:

(a) (i) Na+ , PO43- [Both must be correct]
(ii) Sodium hydroxide [NaOH will NOT be accepted]
(iii) 3NaOH + H3PO4 –> Na3PO4 + 3H2O
(iv) A fixed volume of phosphoric acid is pipetted into a conical flask and a few of                   drops of methyl orange/ phenolphthalein is added. From a burette, sodium hydroxide is       added to the acid until methyl orange turns orange/phenolphthalein turns pink. The             volume of the alkali used is noted.
The experiment is repeated to achieve consistent results. In the absence of the                         indicator, add the average volume of alkali required and a fixed volume of acid used to           obtain a solution of sodium.
Warm the solution of sodium phosphate to obtain a saturated solution
Leave the solution to cool and crystals of sodium phosphate will be obtained.

(b) CuCOM3.Cu(OH)2 ⇌ 2CuO + CO2 + H2O

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Author: matharena

Math Educator

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