14. Answer the following questions on the experimental methods you used. a) (2 p
ID: 1041965 • Letter: 1
Question
14. Answer the following questions on the experimental methods you used.
a) (2 pts) You determined the mass percent of NH3 in Ni(NH3)nCl2 by a back titration. Explain TWO reasons why a direct titration gives inaccurate results.
b) (2 pts) Describe how to check a solution for completeness of precipitation.
c) (2 pts) Explain why you generated H2S in situ, instead of using commercial H2S gas as the sulfide source. And complete the following reaction equation involved in this generation.
CH3CSNH2(aq) + 2 H2O(l) -->
Explanation / Answer
a) Direct titration gives inaccurate results. This happens due to following types of errors
End point is not identical with equivalence point and colour changes of indicators are not instant. Misjudging the colour of the indicator near the end point occurs because different people have different colour sensitivity. Using solution of wrong concentration also create problem is identifying end point of titration. Using wrong amount of indicator also gives inaccurate results.
b) Completeness of precipitation can be identified by following way
The precipitate in the tube is centrifuged and few drops of precipitating reagents is added through the walls of test tube. If more precipitate formed when drops make contact with supernatant liquid the the precipitation is not complete. Centrifugation is important here because sometimes the precipitate formed is soluble in added precipitating reagent, so to ensure complete precipitation added raegent drops must be make contact only with supernatanty liquid.
It can also be done by separating solid precipitate and supernatant liquid from each other and then add the precipitating reagent to the liquid. If again ppt is formed the precipitation is not complete.
c) H2S gas is highly toxic and flamable gas. It quickly deadens the sense of smell. Hydrogen sulphide has the potential to harm both aquatic organism and terestrial plants whe exposed at low concentration. So by considering its harmful effects it can be prepared in situ whenever needed.
2 CH3CSNH2 + 2H2O ----> 2 CH3CONH2 + 2H2S
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