Yacoubb's Chemistry 3/4 Thread

[Yacoubb]

1.00g sample of organic compound known to contain only carbon, hydrogen and oxygen was burned in excess oxygen. 1.91g of carbon dioxide and 1.17g of water are produced. Determine the empirical formula of this compound.

I got C2HO6, but I know that's incorrect. Help would be appreciated!

[lzxnl]

1.00g sample of organic compound known to contain only carbon, hydrogen and oxygen was burned in excess oxygen. 1.91g of carbon dioxide and 1.17g of water are produced. Determine the empirical formula of this compound.

I got C2HO6, but I know that's incorrect. Help would be appreciated!

Number of moles of CO2 = number of moles of carbon in compound
Number of moles of H2O = half the number of moles of hydrogen in compound
Find the mass of the carbon and oxygen atoms, subtract from 1 gram, rest is oxygen.

So...1.91 g CO2 => 1.91/44 = 4.34*10^-2 moles of carbon atoms
1.17 g H2O => 1.17/18 = 6.5*10^-2 moles of water molecules => 1.3*10^-1 moles of hydrogen atoms
Ratio of C:H is approximately 1:3
Now for the oxygen
4.34*10^-2 moles of carbon + 1.3*10^-1 moles of hydrogen have a mass of 4.34*10^-2*12 + 1.3*10^-1 = 0.65 grams
Mass of oxygen is thus 0.35 grams (must sum to 1 gram)
Number of moles of oxygen =  0.35/16 = 2.2*10^-2 moles
Which is very roughly equal to half of the number of carbon atoms
So we have two carbons, six hydrogens and one oxygen. C2H6O

Perhaps you confused hydrogen with oxygen?

[Yacoubb]

Number of moles of CO2 = number of moles of carbon in compound
Number of moles of H2O = half the number of moles of hydrogen in compound
Find the mass of the carbon and oxygen atoms, subtract from 1 gram, rest is oxygen.

So...1.91 g CO2 => 1.91/44 = 4.34*10^-2 moles of carbon atoms
1.17 g H2O => 1.17/18 = 6.5*10^-2 moles of water molecules => 1.3*10^-1 moles of hydrogen atoms
Ratio of C:H is approximately 1:3
Now for the oxygen
4.34*10^-2 moles of carbon + 1.3*10^-1 moles of hydrogen have a mass of 4.34*10^-2*12 + 1.3*10^-1 = 0.65 grams
Mass of oxygen is thus 0.35 grams (must sum to 1 gram)
Number of moles of oxygen =  0.35/16 = 2.2*10^-2 moles
Which is very roughly equal to half of the number of carbon atoms
So we have two carbons, six hydrogens and one oxygen. C2H6O

Perhaps you confused hydrogen with oxygen?

Thank you

But how do you know that the number of mols of CO2 = number of mols of C?

[datfatcat]

Thank you

But how do you know that the number of mols of CO2 = number of mols of C?

End result is carbon dioxide and water. There is no carbon in water (H2O) and hence all carbon atoms go to carbon dioxide (CO2)

[Yacoubb]

End result is carbon dioxide and water. There is no carbon in water (H2O) and hence all carbon atoms go to carbon dioxide (CO2)

I see. Thanks so much!

[Yacoubb]

A few questions:

* Would some factors to consider with wet methods of chemical analysis (gravimetric and volumetric analysis) be that while it requires cheap apparatus, accuracy of data obtained is dependent upon the experimenter's accuracy? Also, for dry methods like chromatography and spectroscopy, while apparatus is expensive, isn't the process of data collection much faster & more accurate.

Gravimetric analysis:
- A precipitate is made by adding excess of a solution to ensure full precipitation of the ion under analysis.
- The precipitate is washed with distilled water to remove impurities and residue.
- The precipitate is then dried, and heated until a constant mass is reached. This ensures all the water has evaporated from the precipitate.

^^ Is that correct? I just want to polish up on my notes for this area. Thanks

[lzxnl]

A few questions:

* Would some factors to consider with wet methods of chemical analysis (gravimetric and volumetric analysis) be that while it requires cheap apparatus, accuracy of data obtained is dependent upon the experimenter's accuracy? Also, for dry methods like chromatography and spectroscopy, while apparatus is expensive, isn't the process of data collection much faster & more accurate.

Gravimetric analysis:
- A precipitate is made by adding excess of a solution to ensure full precipitation of the ion under analysis.
- The precipitate is washed with distilled water to remove impurities and residue.
- The precipitate is then dried, and heated until a constant mass is reached. This ensures all the water has evaporated from the precipitate.

^^ Is that correct? I just want to polish up on my notes for this area. Thanks

Gravimetric: sometimes some of the precipitate can dissolve. For instance, although you can form a precipitate of calcium hydroxide, it is still relatively soluble in water.
Volumetric: titres have a relatively limited accuracy due to the number of sig figs obtainable from the volume. Sure, yes, the factors you presented are relevant, but there are factors that must be considered too. For instance, reaction rate and chemical stability and reactivity (if it reacts significantly with water then neither of gravimetric or volumetric may work viably) may determine whether these "wet" methods are viable.

TLC is faster but inaccurate.
GLC and HPLC are used for slightly different scenarios and both of those are much more accurate than gravimetric. Gravimetric can be faster though. Spectroscopy has other uses, such as determining the identity of compounds and their molecular structures. Chromatography can separate substances; titrations and gravimetric can't really (gravimetric just isolates one particular substance)

As for your gravimetric analysis notes, you forgot about filtering.

[Yacoubb]

Gravimetric: sometimes some of the precipitate can dissolve. For instance, although you can form a precipitate of calcium hydroxide, it is still relatively soluble in water.
Volumetric: titres have a relatively limited accuracy due to the number of sig figs obtainable from the volume. Sure, yes, the factors you presented are relevant, but there are factors that must be considered too. For instance, reaction rate and chemical stability and reactivity (if it reacts significantly with water then neither of gravimetric or volumetric may work viably) may determine whether these "wet" methods are viable.

TLC is faster but inaccurate.
GLC and HPLC are used for slightly different scenarios and both of those are much more accurate than gravimetric. Gravimetric can be faster though. Spectroscopy has other uses, such as determining the identity of compounds and their molecular structures. Chromatography can separate substances; titrations and gravimetric can't really (gravimetric just isolates one particular substance)

As for your gravimetric analysis notes, you forgot about filtering.

Thank you

So with precipitate:
- Add excess of reagent that causes precipitation to ensure the full precipitation of ion under analysis.
- Filter the precipitate.
- Wash the precipitate with cold, de-ionized water to remove impurities and residue.
- Heat the precipitate, and heat until constant mass, to ensure full evaporation of water from the precipitate mass.

^^ Is that better?

Also, what do these stand for: TLC, GLC and HPLC?

[Scooby]

Thank you

So with precipitate:
- Add excess of reagent that causes precipitation to ensure the full precipitation of ion under analysis.
- Filter the precipitate.
- Wash the precipitate with cold, de-ionized water to remove impurities and residue.
- Heat the precipitate, and heat until constant mass, to ensure full evaporation of water from the precipitate mass.

^^ Is that better?

Also, what do these stand for: TLC, GLC and HPLC?

When you wash the precipitate with deionised water most of the impurities you remove will be water-soluble.

TLC = Thin layer chromatography, HPLC = High performance liquid chromatography, GLC = Gas liquid chromatography

[Yacoubb]

When you wash the precipitate with deionised water most of the impurities you remove will be water-soluble.

TLC = Thin layer chromatography, HPLC = High performance liquid chromatography, GLC = Gas liquid chromatography

Thank you

[Yacoubb]

Could someone please check whether my terminology (definitions) are accurate:

* Aliquot - the volume of solution delivered by a pipette.
* Titre - the volume of solution delivered by a burette.
* Standard solution - a solution with a known concentration.
* Equivalence point - the point at which reactants are in stoichiometrically equal amounts.
* End point - the point at which the indicator changes colour.

Also, in regards to gravimetric analysis, would you wash the precipitate with de-ionised water prior to filtering? Because if the precipitate mass contains soluble impurities, the deionized water can be used to wash away these impurities, and then filter it out. The precipitate is then dried, and heated until a constant mass is obtained (i.e. all water has evaporated from the precipitate).

[lzxnl]

Could someone please check whether my terminology (definitions) are accurate:

* Aliquot - the volume of solution delivered by a pipette.
* Titre - the volume of solution delivered by a burette.
* Standard solution - a solution with a known concentration.
* Equivalence point - the point at which reactants are in stoichiometrically equal amounts.
* End point - the point at which the indicator changes colour.

Also, in regards to gravimetric analysis, would you wash the precipitate with de-ionised water prior to filtering? Because if the precipitate mass contains soluble impurities, the deionized water can be used to wash away these impurities, and then filter it out. The precipitate is then dried, and heated until a constant mass is obtained (i.e. all water has evaporated from the precipitate).

Yes, all of the above is correct from my knowledge, except maybe the washing. From my experience, I've always washed it DURING the filtering, but I'm not sure how much of a difference it really makes.

[Yacoubb]

Yes, all of the above is correct from my knowledge, except maybe the washing. From my experience, I've always washed it DURING the filtering, but I'm not sure how much of a difference it really makes.

Thank you
Yeah that sounds right; maybe have the precipitate on filter paper, on a funnel, and then wash it with deionized water, so soluble impurities and other residue are washed off the precipitate.

[Yacoubb]

A student wishes to prepare 500mL of a standard solution of any base of concentration 0.25 M. Would it be better to prepare the solution using solid sodium hydroxide or anyhydrous sodium carbonate?

How would I go about answering that? Thanks

[brightsky]

Solid NaOH is hygroscopic (i.e. it absorbs water from the atmosphere) and so is unsuitable for use as a primary standard (recall the definition of a primary standard). Anhydrous sodium carbonate, on the other hand, has all the properties of a primary standard, and so is infinitely more suitable.