Problem 18 - Chemistry Matter and Change

Question

What is the molarity of a bleach solution containing 9.5 \(\mathrm{g}\) of \(\mathrm{NaOCl}\) per liter of bleach?

Step-by-Step Solution

Answer

The molarity of the bleach solution is approximately 0.1276 M.

Step 1: Understanding Molarity

Molarity is a measure of concentration expressed as moles of solute per liter of solution. We need to convert the mass of NaOCl into moles to find the molarity.

Step 2: Calculate Molar Mass of NaOCl

Determine the molar mass of NaOCl by summing the atomic masses: Na (22.99 g/mol), O (16.00 g/mol), and Cl (35.45 g/mol). Thus, the molar mass of NaOCl is 22.99 + 16.00 + 35.45 = 74.44 g/mol.

Step 3: Convert Mass to Moles

Use the formula: Moles = Mass (g) / Molar Mass (g/mol). For NaOCl, Moles = 9.5 g / 74.44 g/mol, which equals approximately 0.1276 moles.

Step 4: Calculate Molarity

Knowing the solution volume is 1 liter, use the formula: Molarity (M) = Moles of solute / Volume of solution (L). Thus, Molarity = 0.1276 moles / 1 L = 0.1276 M.

Key Concepts

Concentration

Concentration refers to the amount of a substance (solute) present in a certain volume of solution. It gives us an idea of how 'crowded' the solute particles are in the solvent, like water in our everyday life. A more concentrated solution means more solute particles are packed into the same space, yielding a stronger effect, such as a more potent bleach. In chemistry, there are several ways to express concentration, with molarity being one of the most common.

• Molarity (M) indicates how many moles of solute exist per liter of solution.
• Hence, when asked about the molarity of a solution, you're essentially trying to find out its concentration in moles per liter.
• This measure helps in standardizing reactions in labs since it reflects not just the presence but the precise quantity of a solute in a reaction.

Moles

Moles are a fundamental concept in chemistry that helps us count atoms, molecules, and other particles. The mole is a standard unit in chemistry, defined as exactly 6.022 x 10²³ particles, which is Avogadro's number. Rather than counting each tiny particle individually, we use moles to group them into more manageable amounts.

• Using moles, you can relate the amount of a subtance's mass to the number of atoms or molecules it contains.
• When converting a mass of a substance to moles, you need to know the substance's molar mass as this acts like a bridge between atoms/molecules and the real-world sample masses of substances.
• In our solution, understanding how many moles of NaOCl are present helps us determine how concentrated the solution is, displaying the connection between mole calculations and solution concentration.

Molar mass

Molar mass is the mass of one mole of a substance, usually expressed in grams per mole (g/mol). To determine the molar mass of a compound like NaOCl (sodium hypochlorite), you sum the atomic masses of each element within the compound.

• The atomic mass of sodium (Na) is 22.99 g/mol, oxygen (O) is 16.00 g/mol, and chlorine (Cl) is 35.45 g/mol.
• Adding up these atomic masses gives us the molar mass for NaOCl: 22.99 + 16.00 + 35.45 = 74.44 g/mol.
• This figure is crucial as it allows chemists to convert a given mass of NaOCl into moles, which they can then use to find the molarity of a solution.

Remember, knowing the molar mass lets you effectively convert between the mass of a substance and the number of moles, which is vital in chemical calculations.

NaOCl

NaOCl stands for sodium hypochlorite, which is the principal ingredient in bleach. As a powerful disinfectant, it's used for cleaning and sanitizing, but its effectiveness depends heavily on its concentration, which one determines in molarity.

• In household bleach, NaOCl is usually mixed with water to form a solution.
• The concentration of NaOCl affects how well it can perform tasks such as killing bacteria and viruses, so it’s essential to calculate its molarity accurately.
• Understanding the purity and concentration of bleach ensures safe and effective use in both household and industrial settings.

By converting the mass of NaOCl to moles and finding the molarity, you predict how the bleach will behave in various applications, which is valuable in creating effective cleaning solutions.