Question

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?

Answer 1

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.