Question

Given the reaction: Ca + 2H2O → Ca(OH)2 + H2 What is the total number of moles of Ca needed to react completely with 4.0 moles of H2O?

Answer 1

Answer: The number of moles of calcium metal required is 2.0 moles.

Explanation:

We are given:

Moles of water reacted = 4.0 moles

The given chemical reaction follows:

$Ca+2H_2O\rightarrow Ca(OH)_2+H_2$

By Stoichiometry of the reaction:

2 moles of water is reacted with 1 mole of calcium metal

So, 4.0 moles of water will react with = $\frac{1}{2}\times 4.0=2.0mol$ of calcium metal

Hence, the number of moles of calcium metal required is 2.0 moles.

Answer 2

Since the given equation is balanced, we know the Ca and the H2O are in a 1:2 ratio (to see the ratio, look at the coefficients in front of the molecules). Then, there are a few different ways to solve the problem. I am going to show you how to solve it using a ratio of moles of Ca/moles of H2O: x/1=4/2. First, you cross multiply to get 2x=4. Then, you divide both sides by 2 to get x=2 moles of Ca.