# In a lab 178.8g of water is separated into hydrogen gas and oxygen gas. The hydrogen gas has a mass of 20.0g. What is the mass of the oxygen gas produced?

Nov 7, 2016

#### Answer:

Given complete dissociation, I can automatically say that $158.8 \cdot g$ of dioxygen gas were produced.

#### Explanation:

Of course, we could do this analytically, and write the chemical equation for water decomposition, and then apportion molar quantities:

${H}_{2} O \left(l\right) \rightarrow {H}_{2} \left(g\right) + \frac{1}{2} {O}_{2} \left(g\right)$, but why should I bother?

ALL chemical equations conserve mass. There were approx. $180 \cdot g$ of water. If this mass were decomposed and gave $20 \cdot g$ dihydrogen gas, necessarily, there must be $160 \cdot g$ dioxygen gas.

Would the given reaction be exothermic or endothermic? Why??