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?

1 Answer
anor277
Nov 7, 2016

Given complete dissociation, I can automatically say that #158.8*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_2O(l) rarr H_2(g) + 1/2O_2(g)#, but why should I bother?

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

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

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