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Density is a measure of how much mass is contained in a given volume. It is typically expressed in units like grams per milliliter (g/mL) or kilograms per liter (kg/L). To pick an example from everyday life, think of density as comparing a heavy rock and a light piece of foam. Even if they are the same size in volume, the rock is heavier because it is denser.

In our exercise, the density of the ammonia solution is given as \(0.90 \, \mathrm{g/mL}\). This means that every milliliter of the solution weighs 0.90 grams. Knowing the density is crucial because it allows us to convert between mass and volume—an essential step in calculating the molarity of the solution.

To find the volume of the solution, you simply divide the mass by the density. This gives us a sense of how much space the solution occupies, which is important for understanding its concentration in terms of molarity.

Percent by mass, also known as mass percent, is a way to express the concentration of a component in a mixture. It tells you what fraction of the total mass of the solution is made up by the solute. If you imagine a pie chart, percent by mass is like a slice of that pie representing the component.

In our example, the ammonia solution is stated to be 28% NH₃ by mass. This means in a sample of 100 grams of the solution, 28 grams are ammonia. The remaining 72 grams would be the other components of the solution, typically water in an aqueous solution.

Understanding percent by mass helps us determine how much of a solute is in our mixture, providing another piece of the puzzle necessary for calculating molarity, which is a volume-based concentration measure.

An aqueous solution is a type of solution where the solvent is water. Water is often referred to as the "universal solvent" because it can dissolve a wide variety of substances, making these kinds of solutions quite common in chemistry.

In our problem, we have an aqueous ammonia solution. This means that ammonia is dissolved in water. "Aqueous" comes from "aqua", the Latin word for water, signifying the presence of water in the mixture.

Aqueous solutions are major in chemistry because they allow reactions to occur between dissolved substances. In computational problems, they provide a base for various concentration measures, such as molarity. Knowing your solution is aqueous helps understand the environment in which the solute is dispersed.

Molar mass is a conversion factor that allows us to turn grams into moles, facilitating the calculations required for many chemical problems, including determining molarity. It is the mass of one mole of a substance and is usually expressed in grams per mole (g/mol).

In this exercise, we used the molar mass of ammonia (NH₃) which is approximately 17.03 g/mol. This value is derived from the periodic table by adding up the atomic masses of nitrogen and hydrogen that make up ammonia.

To calculate moles from mass, you use the formula:

• Moles = Mass (in grams) / Molar Mass (g/mol)

Using this formula, the 28 grams of NH₃ in the solution were converted to moles, providing the information needed to further calculate the molarity of the solution.