Question

A chemist combines 12 grams of hydrogen with 104 grams of oxygen in an explosive reaction that forms water as its sole product. All of the hydrogen reacts but 8 grams of oxygen remain. How many grams of water are formed?

Short answer

108 grams of water are formed.

Step-by-step solution

Calculate the Mass of Oxygen That Reacts

To find the mass of oxygen that reacts with the hydrogen, subtract the mass of the leftover oxygen from the initial mass of oxygen. Using the equation: Mass of reactive oxygen = Initial mass - Leftover mass.

Determine the Mass of Water Produced

To find the mass of water produced, add the mass of hydrogen that reacted to the mass of oxygen that reacted. The mass of water produced will be the sum of the mass of hydrogen and the reactive mass of oxygen.

Express the Final Result

State the calculated mass of water which is the combined mass of hydrogen and reactive oxygen.

Key concepts

Chemical Reaction

When we talk about a chemical reaction, we refer to the process where substances, known as reactants, transform into new substances called products. Imagine baking a cake—once you mix the ingredients and bake them, you can't easily separate them back into flour, sugar, or eggs; they've become something new, a cake. Similarly, in our exercise, hydrogen and oxygen are our 'ingredients' that, when combined in specific conditions, react to form water, which is entirely different from the separate hydrogen and oxygen molecules.

In a chemical reaction, the reactants undergo a change at the molecular level that rearranges the atoms to create new compounds. The exercise mentions an 'explosive reaction', which indicates the reaction is exothermic, meaning it releases energy. This energy release can sometimes be witnessed in the form of an explosion or the production of heat and light.

Law of Conservation of Mass

The law of conservation of mass states that, in a closed system, mass is neither created nor destroyed during a chemical reaction. In other words, the mass of the reactants must equal the mass of the products. This is comparable to saying that if you have a certain number of lego bricks to build with, you can make many different structures, but you're always working with the same number of bricks—none disappear or magically appear.

In the given exercise, we initially had 12 grams of hydrogen and 104 grams of oxygen. Even though 8 grams of oxygen didn't react, the law ensures that if we tally the mass of the leftover oxygen, the mass of the hydrogen and the mass of the water produced, the sum will be equal to the starting 116 grams (12 grams of hydrogen + 104 grams of oxygen). Understanding this concept is crucial because it helps us to keep track of all the materials before and after the reaction and thereby correctly calculate the mass of the produced water.

Mole Concept

The mole concept is a cornerstone in chemistry that provides a bridge between the microscopic world of atoms and molecules and the macroscopic world we experience. One mole is defined as the amount of a substance that contains as many elementary entities (like atoms or molecules) as there are atoms in 12 grams of carbon-12. This amount is known as Avogadro's number, which is roughly 6.022 x 10²³.

Why is the mole concept important in our exercise? Because it allows us to calculate the amount of reactants and products in a chemical reaction in a more tangible way—grams instead of countless individual atoms or molecules. For instance, by knowing how many moles of hydrogen react with oxygen, we can predict the amount of water produced. Although the original exercise doesn't directly ask us to use moles, understanding this concept can help students to tackle more complex problems where they need to convert grams to moles or vice versa, to properly balance chemical equations.