Final answer:
The average molarity of the commercial bleach before dilution is calculated by first determining the moles of NaOCl in the diluted bleach and then multiplying by the dilution factor. The molarity of the undiluted bleach is found to be 2.8368 M.
Step-by-step explanation:
To calculate the average molarity of commercial bleach before it was diluted, we will use the information provided regarding the concentration of sodium hypochlorite (NaOCl) and the dilution factor. Initially, a 25.00 mL sample of bleach was diluted to 1000 mL. If we know the mass percentage of NaOCl in the bleach is 5.28% (w/v), we can say that in 1000 mL of bleach (which is 1 L), there would be 5.28 g of NaOCl. Since molarity (M) is defined as moles of solute per liter of solution (mol/L), we need to convert grams of NaOCl to moles using its molar mass (74.44 g/mol).
First, calculate the moles of NaOCl: Moles = mass (g) / molar mass (g/mol) = 5.28 g / 74.44 g/mol = 0.07092 mol. Since this is in 1 L of solution, the molarity of the undiluted bleach is 0.07092 M before dilution. Considering that the original concentration was obtained by diluting by a factor of 40, we then multiply the molarity of the diluted bleach by this factor to find the concentration before dilution: 2.8368 M.