# 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?

Jun 30, 2016

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

#### Explanation:

$\text{Moles of carbon dioxide}$ $=$ $\frac{1.32 \cdot g}{44.0 \cdot g \cdot m o {l}^{-} 1}$ $=$ $0.0300 \cdot m o l$.

$\text{Moles of water}$ $=$ $\frac{2.70 \cdot g}{18.01 \cdot g \cdot m o {l}^{-} 1}$ $=$ $0.150 \cdot m o l$ $\left(i\right)$

If there were $0.0300 \cdot m o l$ of carbon dioxide, there were $0.0300 \cdot m o l$ carbon, i.e. $0.0300 \cdot m o l \times 12.011 \cdot g \cdot m o {l}^{-} 1 = 0.36 \cdot g$ $\left(i\right)$

Likewise there were $2 \times 0.150 \cdot m o l$ hydrogen (as the element!),

$2 \times 0.150 \cdot m o l \times 1.00794 \cdot g \cdot m o {l}^{-} 1$ $=$ $0.30 \cdot g$ $\left(i i\right)$

We add $\left(i\right)$ and $\left(i i\right)$ $=$ $0.36 \cdot g + 0.30 \cdot g = 0.66 \cdot g$

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 $0.66 \cdot g$, 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.