Re: q u e s t i o n s -- Chemistry

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Date Posted: 16:51:40 11/05/01 Mon
Author: Mr. Yeo Woei Ter
Subject: Re: q u e s t i o n s
In reply to: tze may 's message, "q u e s t i o n s" on 05:20:41 11/03/01 Sat

Ok, here's the answer:

1) Washing soda is sodium carbonate decahydrate Na₂CO₃.10H₂O, large translucent crystals.
On exposure to air, the crystals loss mass and become coated with a fine powder which render them opaque. Each molecule of washing soda has given up to the atmosphere 9 molecules of water of crystallisation.
Na₂CO₃.10H₂O(s) ---> Na₂CO₃.H₂O(s) + 9H₂O(g)

2) Yee Lian is correct for this question. The equations are in your "*textbook!", try to search for it.

3) Yee Lian is correct, note that Barium hydroxide is also soluble.

4) I think I have already replied to this question. Calcium sulphate formed in the second reaction is only slightly soluble. So it will form a layer on the surface of the carbonate and prevent it from further reaction with the acid.

5)The equations for the reactions are:
2KOH + H₂SO₄ ---> K₂SO₄ + 2H₂O

KOH + H₂SO₄ ---> KHSO₄ + H₂O

From the equations above, it is evident that the amount of potassium hydroxide needed to convert a given amount of sulphuric acid into potassium hydrogencarbonate is half that required to convert it to potassium carbonate.

Firstly, measure say, 25cm³ of 1 mol/dm³ potassium hydroxide solution into a titration flask, add indicator and then run carefully, from a burette, 1 mol/dm³ dilute sulphuric acid, until the end point is reached. Note the volume of dilute sulphuric acid needed (say x cm³). The solution contains potassium sulphate.

Then, measure out 25cm³ of the same potassium hydroxide solution and add to it from the burette, 2x cm³ of the same acid. This solution now contains potassium hydrogensulphate.

(Then you must describe how to obtain the crystals from the solution by the usual way)

6) Yes, the more reactive the metal, the stronger its reducing property.

7) Yee Lian is right.

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