One brand of laundry bleach is an aqueous solution containing 4.55% sodium hypochlorite (NaOCl) by mass. What is the molarity of this solution? (Assume a density of 1.02 g>mL.)

Understanding the concentration of a solute like sodium hypochlorite in a solution is crucial for various applications, including laundry bleaching. Concentration can be expressed in several ways, including by mass percent. In the context of our exercise, the concentration is given as 4.55% sodium hypochlorite by mass, which means that in every 100 grams of the bleach solution, there are 4.55 grams of sodium hypochlorite.

When you calculate the molarity, which is the number of moles of solute per liter of solution, you're moving from a mass-based concentration to a volume-based one. This shift requires information about the solution's density and the solute's molar mass to accurately convert mass percent to molarity.

The mass percent solution is a common expression of concentration in chemistry, indicating the mass of solute in grams per 100 grams of solution. To elucidate, if a solution has a 4.55% mass percent of sodium hypochlorite, it contains 4.55 grams of NaOCl for every 100 grams of the overall solution.

To use mass percent for molarity calculations, you first need to find the mass of the solute in a specific volume of solution—often using the solution's density—followed by converting that mass to moles using the molar mass of the solute.

Solution density plays a vital role in converting mass percent to molarity. Density is defined as mass per unit volume, typically expressed in grams per milliliter (g/mL) for liquid solutions. In our exercise, the bleach solution has a density of 1.02 g/mL. This means each milliliter of solution has a mass of 1.02 grams. With this information, along with the mass percent, we can determine the mass of sodium hypochlorite in any volume of the bleach solution. This step is necessary because molarity is concerned with moles per liter; hence we need to know how much solute is in a liter to calculate it.

Molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). It is a bridge between the macroscopic world (grams) and the atomic or molecular scale (moles). For sodium hypochlorite (NaOCl), the molar mass is 74.44 g/mol.

The molar mass allows us to convert the mass of NaOCl we found in a milliliter of solution into moles, which is a necessary step for calculating the molarity. As in our exercise, we divided the mass of NaOCl per milliliter by the molar mass to find the number of moles in that volume. These moles can then be scaled up to a liter to find the molarity, which is a measure of how concentrated the NaOCl is in the bleach solution.