Homework Help: Chemistry

Posted by candy on Sunday, April 5, 2009 at 6:19am.

1) Bases that are soluble in water are known as alkalis. Is sodium oxide an alkali?

2) Indicators change colour at different pH. E.g. methyl orange changes colour in the pH range 3-5 while phenolphthalein changes colour in the pH range 8-10. How do I know which indicator to use?

Chemistry - DrBob222, Sunday, April 5, 2009 at 1:50pm
1. Yes.
2. Use the indicator that changes in the rage of pH that is the pH at the equivalence point of a titration/reaction.
For example,
Na2CO3 + HCl ==> NaHCO3 + NaCl
The equivalence point is about 8.3 for th addition of ONE H so you would use phenolphthalein for this equivalence point. However, if we add two H atoms,
Na2CO3 + 2HCl ==> H2O + CO2 + 2NaCl,
the pH at the equivalence point is about 4.5-5 or so and methyl orange or methyl red can be used. Phenolphthalein can not be used for this part of the titration. You may be interested in knowing, if you don't already, that mixtures of NaHCO3 and Na2CO3 can be titrated in two steps of the same titration. First we add phenolphthalein, which turns pink, and titrate to the colorless end point which is the addition of one H atom to the Na2CO3. Then we add a few drops of methyl red (the phenolphthalein is colorless so it doesn't interfere on the acid side) and titrate to the second end point, which is the addition of two H atoms. It titrates all of the NaHCO3 formed in the first part of the titration (from Na2CO3) plus all of the NaHCO3 that was in the original sample but which was not titrated in the first part.

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