Chemistry- Finding Empirical and Chemical Formulae by Experiment, Sept. 15, 2008

Finding Empirical and Chemical Formulae by Experiment

 

Chemists often can determine the percentage composition and molar mass of an unknown through experimentation

                – Using this information they can develop the chemical formula

 

Why is this important?

– we can learn more about compounds and figure out how to synthesize them

ex. Penicillin is commonly used to inhibit bacterial growth

Comes from a mold naturally

One of the biggest challenges was figuring out how to synthesize it- the molecule was sensitive to heat and acid treatments

 Once we could synthesize penicillin, we could make it readily available to save lives and limbs (ex. WW2)

 

Example of method of finding out the composition of a compound: The Carbon-Hydrogen Analyzer

– we can use to analyze compounds made of carbon, oxygen and hydrogen

 

One burns the unknown compound in a furnace and passes a stream of pure O2 through it

The compound breaks down into carbon dioxide and into H2O vapour which are collected separately by 2 chambers

Water is collected by a magnesium perchlorate tube

The carbon dioxide is collected by a sodium hydroxide tube

 

The tubes are weighed before and after the experiment. The difference in masses will tell us how much water and how much carbon dioxide was collected.

 

The tube that collected the water will tell us how much water was produced from the incineration and therefore tell us how much hydrogen was present

 

The tube that collected the carbon dioxide will tell us how much carbon dioxide was produced from the reaction and therefore tell us how much carbon was present.

 

If oxygen was present in the unknown sample, the mass of the unknown sample minus the mass of CO2 and the mass of H2O will tell us how much oxygen was present.

 

We can use the formulae and the methods we discussed earlier in the section and apply it in experimental situations. We will be doing that in the lab tomorrow.

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