Traditional methods of analysis

Question 1

one method of finding the percentage composition by mass (which then allows you to calculate the empirical formula) is through

Answer 1

combustion analysis

Question 2

the practical method of doing combustion analysis is

Answer 2

burn a known mass of the compound and the masses of carbon dioxide and water formed are found by measuring the increase of mass of U-tubes containing absorbers

Question 3

the steps in calculating the empirical formula from combustion analysis is:
mass of compound X: 1.98g
mass of water formed: 1.44g
mass of carbon dioxide formed: 3.53g

Answer 3

*calculate the masses of C and H2 in the CO2 and H2O
the proportion of C in CO2 is 12/44 while H2 in H2O is (2 x 1.0)/18
(1.44 x 2)/18 = 0.160g H2
(3.53 x 12) / 44 = 0.963g C
*calculate the mass of oxygen present in the compound
add the masses of H2 and C and subtract this sum from the mass of compound X
1.98 - (0.160 + 0.963) = 1.98 - 1.123 = 0.857g O
*calculate the empirical formulae using a table format to work out the ratios
C 0.963 ÷ 12.0 = 0.0803 = 1.5
H 0.160 ÷ 1.0 = 0.160 = 3
O 0.857 ÷ 16.0 = 0.0536 = 1

whole number ratio: 3:6:2 C3H6O2

If percentage compositions by mass are given, and they do not add up to 100%, then the remainder should be Oxygen

Question 4

to determine the molecular from the empirical formula, you need to :

Answer 4

divide the molar mass of the compound by the ‘molar mass’ of the empirical formula to see if you need to multiply the empirical formula by something to obtain the molecular formula

Question 5

the test for a COOH group is…………. and the positive result is…………….

Answer 5

• add sodium hydrogen carbonate

- effervescence

Question 6

the test for a C=O group is…………. and the positive result is…………….

Answer 6

• add 2,4-dinitrophenylhydrazine solution

- orange-yellow precipitate forms

Question 7

the test for a CHO (aldehyde) group is…………. and the positive result is…………….

Answer 7

-add Tollen’s reagent and warm

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Question 8

the test for a compound that can be oxidised (could be an OH group) is…………. and the positive result is…………….

Answer 8

• add acidified potassium dichromate(VI) and heat

- colour change from orange to green