An hydrocarbon with a #0.66*g# mass was completely combusted to give a #1.32*g# mass of carbon dioxide, and a #2.70*g# of water. How do these data support the idea of the conservation of mass?

1 Answer
Jun 30, 2016

Answer:

Compare the mass of carbon and hydrogen PRIOR to, and AFTER the combustion. Is mass conserved?

Explanation:

#"Moles of carbon dioxide"# #=# #(1.32*g)/(44.0*g*mol^-1)# #=# #0.0300*mol#.

#"Moles of water"# #=# #(2.70*g)/(18.01*g*mol^-1)# #=# #0.150*mol# #(i)#

If there were #0.0300*mol# of carbon dioxide, there were #0.0300*mol# carbon, i.e. #0.0300*molxx12.011*g*mol^-1=0.36*g# #(i)#

Likewise there were #2xx0.150*mol# hydrogen (as the element!),

#2xx0.150*molxx1.00794*g*mol^-1# #=# #0.30*g# #(ii)#

We add #(i)# and #(ii)# #=# #0.36*g+0.30*g=0.66*g#

Now we added oxygen to the combustiion, but we did not add further carbon and hydrogen. The mass of #C# and #H# in the combustion products were #0.66*g#, precisely equivalent to the mass of #C# and #H# in the starting material. Mass has been conserved, and if it were not I would know I had made a mistake.