# STOICHIOMETRY

## Stuck on Mars

In the book The Martian, astronaut Mark Watney is left behind by accident. Fortunately, he learned a lot about chemistry in school, so he was able to figure out how to make the water he needed to survive.

Hydrogen and Oxygen are gas, but if they are mixed together and a spark is added, they make liquid water.

## A limiting reactant problem example: What mass of water can be produced by 4 grams of hydrogen gas reacting with 16 grams of oxygen gas?

1. Write a balanced equation for the reaction.

2 H2 + O2 → 2 H2O

2. Convert both reactant quantities to moles.

3. Using the mole ratio from the equation, determine the moles of water that could be formed by each reactant.
4. Oxygen produces the least amount of water.
• 16 grams of oxygen cannot produce as much water as 4 grams of hydrogen. In other words, 16 grams of oxygen will be used up in the reaction before 4 grams of hydrogen.
• Oxygen is the "limiting" reactant.
• Use oxygen for the calculation of product amount.
5. Complete the problem by converting moles of H2O to mass of H2O.
The theoretical yield for this problem is 18 grams.