EUDIOMETRY (GAS ANALYSIS) :- APPLICATION OF POAC ON EUDIOMETRY (gas analysis) NUMERICALS and HOW TO FIND OUT MOLECULAR FORMULA OF ANY HYDROCARBON HYDROCARBO N USING "POAC" Eudiometry:- Eudiometry means gas analysis. It means in this type of numericals either you have to find out vol. of GAS From the data given in question or you have to find out any other thing like molecular formula of hydrocarbon ,no. of mole , mass of any other reactant and vol. of GAS is given. Specific gases are absorbed in specific reagents as given below Reactants
Absorbed gases
KOH solution
CO2 & SO2
Alkaline Pyrogallol
O2
Solution of ammonical cuprous chloride
CO
And if the mixture is cooled, H 2O vapour changes to liquid. 0
Hence, when temperature is above 100 C (373 K) then only volume of H 2O is considered. Eudiometry is mainly based on Avogadro's law, A (g) +
B (g) → C(g) + D(g)
a volume b volume c volume d volume a moles
b moles
c moles
d moles
Q.1 Three moles of Na2CO3 is reacted with 6 moles of HCl solution. Find out the volume of CO2 gas produced at STP
Sol. Na2CO3→ NaCl + CO2 Apply POAC on C atom
No. of moles of C atoms in Na2CO3 = no. of moles of C atoms in CO 2 1* no. of moles of Na2CO3 = 1* no. of moles of CO 2 So no. of moles of Na2CO3 = no. of moles of CO 2 = 3 moles (given) Vol. of CO2 = 3 * 22.4 litre = 67.2 litre
HOW TO FIND OUT MOLECULAR FORMULA OF ANY HYDROCARBON USING POAC:Q. 10 ml. of a gaseous hydrocarbon was burnt completely in 80 ml. of O 2 at NTP. The remaining gas occupied 70 ml. at NTP. This vol. became 50 ml on treatment with KOH solution. What is the formula of hydrocarbon? Solution: - Cx Hy + O2 → CO2+ H2O Vol. of ( CO2 + unreacted O2) = 70 ml. Because CO2 can be absorbed by KOH . So when this 70 ml. mix. Is treated it becames 50 ml it means vol. of CO 2 in the mixture was 20 ml. which is absorbed by KOH and remaining 50 ml. was unreacted O2 ( O2 can NOT absorbed by KOH) So vol. of CO 2 = 20 ml. Vol. of O2 reacted = 80 -50 = 30 ml. Vol. of hydrocarbon = 10 ml. Apply POAC on C atoms X* no. of mole of C x Hy = 1* no. of mole of CO2 -----------------------( i )
X* 10 = 20 So X = 20/10= 2 Apply POAC on H atoms Y* no. of moles of C x Hy = 2* no. of mole of H 2O -----------------( ii ) Apply POAC on O atoms 2* no. of moles of O 2 = 1* no. of mole of H2O+2* no. of mole of CO2 From eq. (ii) 2* no. of moles of O 2 = y/2* no. of mole of Cx Hy + 2* no. of mole of CO 2 2*30 = Y/2*10 + 2 *20 20 = 10Y/2 Y=4 Put the value of X and Y in C x Hy The molecular formula of hydrocarbon is C 2H4
TRY TO SOLVE FOLLOWING QUESTION
Q.2 7.5 ml of hydrocarbon gas was exploded with excess of oxygen. On cooling it was found to have undergone a contraction of 15 ml. if vapour density of hydrocarbon is 14. Determine its molecular formula.
Solution ( HINT ) :- molecular wt.= 2* vapour density
Answer:- C2H4
Q.3 7.5 ml of hydrocarbon gas was exploded with 36 ml.of oxygen. The vol. of gases on cooling was found to be 28.5 ml., 15 ml. of which was absorbed by KOH and the rest was absorbed in a solution of alkaline pyrogallol. If all vol. are measured under the same conditions, determine the molecular formula of hydrocarbon. Answer:- C2H4
