It is possible to determine the empiricala formula from percent composition of a

Question

It is possible to determine the empiricala formula from percent composition of an unknown: % composition = elemental mass/compound mass x100

Procedure: The following is the procedure by which you can obtain an empirical formula from combustion analysis

1. Mass H2O & CO2 --> Their respective Moles

2. Moles H2O & CO2 --> Moles C & H (from their chemical formulas) 2+ may need to convert moles back to grams

3. Mass O --> subtract mass of C & H (step 2+) from the mass of the original sample. O --> moles.

4. Write down the calculated formula using the numver of moles of each element as the subscripts.

5. Divide all of the subscripts by the smallest subscript to yield the whole number subscript for each element.

Can anyone solve for the equations on the sheet and show me how to do them? It seems so simple, yet I can't understand.

Data Name Compound A is an unknown which upon combustion yielded the following data. Compound A starting weight 2.146 U-tube with CO2 absorber Initial weight 65.137 Final weight after combustion 70.631 U-tube with H20 absorber Initial weight 63.038 Final weight after combustion 65.291 Fill in the following table. Show your calculations on pgs. 3-4. Hydrogen oxygen Carbon H20 CO2 mass x CO2 moles moles mass Proceed to step 4 Table 1

Explanation / Answer

moles of C = moles of CO2 = 5.494/44 = 0.125 mol

mass of C = 0.125 x12 = 1.5 g

moles of H = 2 xmoles of H2O = 2 x (2.253/18) = 0.250 mol

mass of H = 0.250 g

mass of O = 2.146 - (1.5 + 0.25) = 0.396 g

moles of O = 0.396/16 = 0.025 mol

divide moles by smallest number

C = 0.125/0.025 = 5

H = 0.250/0.025 = 10

O = 0.025/0.025 = 1

formula of the compound = C5H10O

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