How do I solve Mg+HCI=MgCl+H2 for stoichiometry mole to mole?

Question

The question asks:

Magnesium reacts with hydrochloride acid in a single replacement reaction. You begin with 6.6 moles of magnesium and an excess of the acid. You performed a bark test for hydrogen and it made a loud noise. How many moles of each product are formed?

Please help me..

Answer 1

Hi Arwen,

The substitution reaction in question between elemental magnesium and hydrochloric acid can be written as:

Mg + HCl --> MgCl + H₂

To balance out the equation, you have to stoichiometrically equate the elements on the left with those on the right. In order to do that, take each group (in this case, there are three: Mg, H, and Cl) and balance their numbers on the left to their numbers on the right:

This is a step-wise process which just requires you to take a step-wise approach.

1. I always start with the first group (in this case, Mg). There is one on the left, and one on the right. So we will keep that number as is.
2. Let's take a look at the Hydrogen (H) in HCl next: there is one hydrogen on the left but there are two on the right. Therefore, we will have to add 2 to the original equation, making it:
3. Mg + 2HCl --> MgCl + H₂
4. Now we check the number of chloride ions (Cl) on each side. There are now two Cl ions on the left. So we will have to double the number of MgCl molecules on the right. This turns the equation to:
5. Mg + 2HCl --> 2MgCl + H₂
6. Because we added another Mg ion to the right side, we will have to balance the Mg ions on the left side as well, making the final equation:
7. 2Mg + 2HCl --> 2MgCl + H₂

Now that we have the proportions of each molecule in the reaction, let's focus on the reactants to determine which will be our limiting reagent. We have two reactants, the magnesium and the hydrochloric acid. The question states that there are only 6.6 moles of magnesium but an excess of acid. This means that we will use the amount of Mg to determine the equivalent amount of moles of all the others.

Since this equation shows 2 equivalents of Mg, we can assume that 2 equivalents of Mg equals to 6.6 moles (M). Therefore, one equivalent of any of the reactants or products in the equation is equal to 3.3 M. This makes the final equation (and the answer):

6.6 M Mg + 6.6 M HCl --> 6.6 M MgCl + 3.3 M H₂