Chemistry qn

[IntoTheNewWorld]

A 0.360g sample of an organic compound containing carbon, hydrogen and oxygen was burnt in excess oxygen. When the gases evolved were passed through anhydrous CaCl2 its mass increased by 0.216g. The remaining gases, when bubbled through a NaOH solution, increased its mass by 0.528 g. The mass of oxygen in the sample is...
A. 0.192
B. 0.144
C 0.024
D. unable to be determined from info given

soooo lost.

[Mao]

where did you get the question from? I'm either interpreting it incorrectly or the answers are incorrect.

[IntoTheNewWorld]

I got it from this worksheet my teacher gave me. Don't know the answer yet though.

[Mao]

here's how I did it:

water + carbon dioxide passes through anhydrous calcium chloride, water gets absorbed into calcium chloride, i.e. 0.216g of water, carbon dioxide passes through.

carbon dioxide passes through sodium hydroxide, gets absorbed into solution [reacts with NaOH], i.e. 0.528g of carbon dioxide

mass of product gases: 0.216 + 0.528 = 0.744g

mass of original fuel: 0.360g

mass of oxygen consumed during burning: 0.744 - 0.360 = 0.384g


When we break down what these numbers actually mean, calculating the moles of C, H and O, we have 0.012mol of C, 0.024mol of H and 0.036mol of O (0.012mol in organic compound, 0.024mol consumed from atmosphere). It could possibly be methanal (an aldehyde, O=CH₂), methyl methanoate (H₃COOCH), glucose (C₆H₁₂O₆), and a range of other things, but all combust to the formula below:



the amount (mass) of oxygen consumed during combustion will always be ~107% of the mass of the organic compound, none of the answers satisfy that.


IGNORING ALL OF THE ABOVE, there are 0.012mol of oxygen in original sample, which is 0.192g

[beezy4eva]

since we assume all the hydrogen in the original sample is converted to H2O which is then absorbed by the CaCl2, we can calculate the mass of H in the original substance:
m(H)= 0.216 x (2.0/18.0) = 0.0240g

we can do the same for the carbon using the weight gain from the CO2 in the NaOH(aq):
m(C)= 0.528 x (12.0/44.0) = 0.144

This gives us a total mass of 0.168g. The remaining mass must be the oxygen, therefore the mass of oxygen in the sample is 0.360 - 0.168 =0.192g ie answer A =D

[Mao]

since we assume all the hydrogen in the original sample is converted to H2O which is then absorbed by the CaCl2, we can calculate the mass of H in the original substance:
m(H)= 0.216 x (2.0/18.0) = 0.0240g

we can do the same for the carbon using the weight gain from the CO2 in the NaOH(aq):
m(C)= 0.528 x (12.0/44.0) = 0.144

This gives us a total mass of 0.168g. The remaining mass must be the oxygen, therefore the mass of oxygen in the sample is 0.360 - 0.168 =0.192g ie answer A =D

oh lord... classical example of Mao not reading the question....

[IntoTheNewWorld]

thanks guys, I think I get it now