[Julie Smith]

hi,

I have a couple of questions in regards to how certain things will effect the empirical formula of a compound, for example Calcium hydroxide. I had a huge list but I've narrowed it down to just a few now.

1. what happens to the empirical formula if the back titration with NaOH is done on a cold solution? So basically, if the dissolved CO2 isnt removed? Would it even have an effect?

2. how about if the concentration of the NaOH used for the back titration is increased?

3. how about if there's a small amount of calcium oxide (solid) present as an impurity or even a small amount of aluminum hydroxide present in the sample as an impurity?

4. Lastly, how about if the calcium hydroxide was left exposed to the atmospere for 30minutes before a sample was weighed on the analytical balance?

If anyone can help, I'd greatly appreciate it.

[Rabn]

1) if you measure the pH of DI water it usually has a pH of about 6.8.  It is slightly acidic because the CO₂ and H₂O form carbonic acid. That is something to keep in mind and the added acidity may effect you calculations. Essentially you should run your calculations assuming a starting pH of 7.0 and then with a starting pH of 6.8; if the difference is more than 5% then it should be considered significant.

2/3)If the conc. of NaOH is increased the change in pH per drop of solution is greater, look up the pH of precipitation of CaOH₂  and Al₂OH₃.  If they are close then using a more concentrated solution of NaOH could ppt. them both simultaneously because the pH threshhold may be crossed without intention.  If CaO is present it would be converted to CaOH₂ in a 1 to 1 ratio.http://scifun.chem.wisc.edu/chemweek/lime/lime.html.  Use the Ksp values of aluminum and calcium hydroxide to calculate the pH threshold of each.

4) Calcium hydroxide is hygroscopic(meaning it will absorb water from the atmosphere) so you may have introduced error into your calculations and is something that should be noted in your lab write up.

[Julie Smith]

So would an acidic solution effect the empirical formula of a compound? because I think adding an excess amount of NaOH would probably decrease the ratio between Ca and OH in calcium hydroxide, so would an acidic solution do the same? Or perhaps have no effect on the Ca(OH)2 compound?

Thanks

[Rabn]

Most metal hydroxides will precipitate out of solution only once the pH goes above a threshold value. Adding excess NaOH won't skew your results because chemicals will only react in a fixed ratio, stoichiometry. If the solution is acidic you may not have ppt.'d out all that you could.  You really need to get the Ksp and do some math to elucidate these questions.