how to do? -- Chemistry

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Date Posted: 03:37:13 10/29/01 Mon
Author: fiona yeo
Subject: how to do?

ca(OH)2- slightly soluble
caSo4- slightly souble
caco3- insouble
koh- soluble
k2(so4)- soluble
k2(co3)-soluble
zn2(oh)-insoluble
znso4-soluble
znco3- insoluble

Qn: how to get zinc carbonate? (T.B pg 157 q:8; mcq)
Ans: shake solid zinc sulphate with water and add aq potasium carbonate.

mr yeo, how to do these sort of questions, what is the steps to follow first??

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[> Re: how to do? -- Mr. Yeo Woei Ter, 01:36:29 10/30/01 Tue

First of all, you need to know if the salt you are preparing is a soluble one or not. (refer to the flow chart in page 151 of "*textbook!")

If the salt is soluble, then, you have to know whether you'll be using a soluble base or an insoluble one. If the base use is a soluble base, ie, an alkali, you need the titration method. If the base used is insoluble, then use the method as described in pages 148 and 149 in Experiment 7.3 and Experiment 7.4(for using insoluble carbonate)of your "*textbook!".

However, if the salt to be produced is insoluble in water, then we need to use the precipitation method. In this method, we mix solutions of two soluble salts to precipitate the insoluble salt.

For the question that you mentioned, zinc carbonate is insoluble in water. So we'll have to use the precipitation method we just discussed. We have to mix solutions of two soluble salts to precipitate the zinc carbonate. One of the soluble salt used must contain zinc ions (ie. a soluble zinc salt) and the other one used must be a soluble carbonate.

From the table given in the question, zinc sulphate is the soluble zinc salt available, and potassium carbonate is also soluble. So the answer is C! Where we shake the solid zinc sulphate in water to dissolve it and then mix it with aqueous potassium carbonate. A white precipiate of zinc carbonate will be formed! The equation for the reaction is :

ZnSO₄(aq) + K₂CO₃(aq) ---> ZnCO₃(s) + K₂SO₄(aq)

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