This worked chemistry example problem demonstrates how to determine the number of reactants needed to complete a reaction in an aqueous solution.

## Problem

For the reaction:

- Zn(s) + 2H
^{+}(aq) → Zn^{2+}(aq) + H_{2}(g)

- Determine the number of moles H
^{+}that is required to form 1.22 mol H_{2}. - Determine the mass in grams of Zn that is required to form 0.621 mol of H
_{2}

- Determine the number of moles H

## Solution

**Part A**: You may wish to review the types of reactions that occur in water and the rules that apply to balance aqueous solution equations. Once you have set them up, balanced equations for reactions in aqueous solutions work in exactly the same way as other balanced equations. The coefficients signify the relative number of moles of substances participating in the reaction.

From the balanced equation, you can see that 2 mol H^{+} is used for every 1 mol H_{2}.

If we use this as a conversion factor, then for 1.22 mol H_{2}:

- moles H
^{+}= 1.22 mol H_{2}x 2 mol H^{+}/ 1 mol H_{2} - moles H
^{+}= 2.44 mol H^{+}

**Part B**: Similarly, 1 mol Zn is required for 1 mol H_{2}.

To work this problem, you need to know how many grams are in 1 mol of Zn. Look up the atomic mass for the zinc from the Periodic Table. The atomic mass of zinc is 65.38, so there is 65.38 g in 1 mol Zn.

Plugging in these values gives us:

- mass Zn = 0.621 mol H
_{2}x 1 mol Zn / 1 mol H_{2}x 65.38 g Zn / 1 mol Zn - mass Zn = 40.6 g Zn

## Answer

- 2.44 mol of H
^{+}is required to form 1.22 mol H_{2}. - 40.6 g Zn is required to form 0.621 mol of H
_{2}