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Question

What volume of Hydrogen gas is produced when 1.23L of 1.50M HCI reacts with Zinc?

Answer 1

—First, write out the reaction:

This is a single displacement reaction, meaning it follows the formula A + BC --> AC + B

We know that hydrogen gas always exists as a bivalent compound (remember H O N Cl Br I F) so one product is H₂ and Zn forms a +2 ion while Cl forms a -1 ion, creating the neutral compound ZnCl₂

Zn + HCl --> H₂ + ZnCl₂

Next, make sure it is balanced:

Zn + 2HCl --> H₂ + ZnCl₂

Now we can solve the question. Always start your calculations with the variable that has units (in our case liters), NOT a conversion factor (M, or moles/liter).

1.23 L HCl

Begin dimensional analysis by using the conversion factor, making sure units will cancel out.

1.23 L HCl x 1.50 moles HCl

L

The next multiplier will be found in the chemical equation. It tells you that 2 moles of HCl are needed to form 1 mol of hydrogen gas.

1.23 L HCl x 1.50 moles HCl x 1 mol H₂

L 2 mol HCl

Molar volume means 1 mol of any gas at STP is 22.4 L in volume, so this is the next conversion factor.

1.23 L HCl x 1.50 moles HCl x 1 mol H₂ x 22.4 L H₂

L 2 mol HCl 1 mol H₂

Make sure your units cancel out

1.23 L HCl x 1.50 moles HCl x 1 mol H₂ x 22.4 L H₂

L 2 mol HCl 1 mol H₂

Plug the numbers into your calculator: 1.23 × 1.50 ÷ 2 × 22.4

= 20.7 L H₂