Zinc and magnesium metal each react with hydrochloric acid to make chloride salts of the respective metals, and hydrogen gas. A \(10.00-\mathrm{g}\) mixture of zinc and magnesium produces \(0.5171 \mathrm{~g}\) of hydrogen gas upon being mixed with an excess of hydrochloric acid. Determine the percent magnesium by mass in the original mixture.

When dealing with chemical reactions, understanding stoichiometry is pivotal. Stoichiometry is the area of chemistry that addresses the quantitative relationships between reactants and products in a chemical reaction. It's like a recipe that tells you how much of each ingredient you need to make a dish, and what you'll get at the end.

For example, let's consider the reaction in our exercise where magnesium (Mg) and zinc (Zn) react with hydrochloric acid (HCl) to produce chloride salts and hydrogen gas (H₂). The stoichiometry of the reaction tells us that each mole of Mg or Zn will produce one mole of H₂. By understanding this, you can determine the amounts of reactants consumed and products formed.

In the initial problem, we wanted to know the percent composition of magnesium in the mixture, so we referred to how many moles of hydrogen gas were produced, and worked backwards through the stoichiometry of the reaction to find the amount of magnesium and zinc that reacted.

The mole is a fundamental concept in chemistry, providing a bridge between the atomic world and the world we can measure. A mole is 6.022 x 10²³ of something, be it atoms, molecules, or ions. When dealing with chemical quantities, the mole allows us to count particles by weighing them.

The mole concept is essential in stoichiometry because reactions occur on a particle level, and we need to use moles to make sense of these reactions on a scale we can observe. By using the molar mass of a substance, which is the weight of one mole of that substance, we can convert between grams and moles, as we did in the exercise with hydrogen gas.

For magnesium, we used its molar mass to convert the mass of magnesium that had reacted into moles. By doing this, we could use the stoichiometric relationships that exist within the balanced equation of the chemical reaction to determine the percentage of magnesium in the original mixture.

In chemistry, the percent composition of a mixture or a compound is a measure of the relative amounts of each substance within it. This tells you how much of each element is present compared to the total mass. Calculating percent composition is straightforward: you divide the mass of the element of interest by the total mass of the compound or mixture, then multiply by 100 to get a percentage.

In the exercise, once we determined the mass of magnesium that had reacted with the acid, we could calculate its percent composition in the original mixture. This was done by dividing the mass of magnesium by the total mass of the combination of zinc and magnesium, and then multiplying by 100 to get the percentage. This percentage helps scientists and chemists understand the makeup of an unknown sample or to verify the purity of a substance.