this acid? YOUR TURN 3: A compound is made up of carbon, hydrogen and oxygen ato

Question

this acid? YOUR TURN 3: A compound is made up of carbon, hydrogen and oxygen atoms. When burned in oxygen, 3.584 g of CO2 and 1.957 g of H2O are formed. The mass of oxygen in the compound is 36.3% that of the CO2 produced. (a) What is the empirical formula of the compound? Answer: C3H8O3 (b) How much sample was burned? Answer: 2.50 g YOUR TURN 4A. Substance X is composed of C, H and O. Combustion of 0.255 g of this substance X yields 0.561 g CO2 and 0.306 g H2O. What is the empirical formula of this substance? Answer: C3H8O YOUR TURN 48. What is the molecular formula of Substance X from above if the molar mass was determined experimentally as being 120.188 g/mol? Answer: C6H16O2

Explanation / Answer

Mole of CO2 = 3.584/44 = 0.08145 mole ; Moles of O2 in CO2 = 2*0.08145 = 0.1683 moles and its mass = 2.6928 g

Moles of water = 1.957/18 = 0.1087 moles; moles of O2 in H2O = 0.1087 moles and its mass = 1.7392g

Mass of Oxygen in compound = 0.363*3.584 = 1.301g

Moles of oxygen consumed from air = (2.6928+1.7392)-1.301 /16 = 0.196 mole

C:H:O ratio in the compound is 0.8145:0.2174:0.0813 => 1:2.667:0.998 => 1:8/3:1 or 3:8:3

Emperical formula is C3H8O3

b)emperical weight of compound = 92, mole of compound burned = 0.08145/3 = 0.02715

mass = 92*0.02715 = 2.5g

4a) mole of CO2 = 0.01275; water = 0.017 moles

mole ratio of C:H = 0.01275 : (2*0.017) => 0.01275:0.034 => 1:2.6667 => 3:8

Therefore, moles of compound X = 0.01275/3 = 0.00425. Molecular mass = 0.255/0.00425 = 60

O = 60 - (36+8) = 16 Therefore the emperical formula is: C3H8O

b) If molecualr mass is 120.188, then the Molecualr formula is = (M.mass/E. mass)*E.formula) = C6H16O2

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