Final answer:
To find the molarity of hypochlorite in bleach, the titration with Na₂S₂O₃ is considered to determine the moles of I₃⁻ which equals the moles of hypochlorite pre-dilution. After considering the dilution factor, the mass and volume of the bleach, the %w/v NaOCl is calculated and represents the concentration in the original sample.
Step-by-step explanation:
To calculate the molarity of the hypochlorite ion in the original, concentrated bleach, we start by calculating the molarity of the I₃⁻ solution using sodium thiosulfate (Na₂S₂O₃). In the provided example, 1₂ drops of 0.01 M Na₂S₂O₃ were needed, but we will use the given volume of 8.96 mL of 0.09892 M Na₂S₂O₃ for a more precise calculation. The moles of Na₂S₂O₃ used in the titration are found by multiplying the volume by the molarity (0.09892 M * 8.96 mL * 1 L/1000 mL).
The moles of Na₂S₂O₃ are equal to the moles of I₃⁻ (since their stoichiometric coefficients are equal in the balanced reaction). Knowing the moles of I₃⁻ and using the original sample volume, we can find the molarity before the dilution. Since the bleach was initially diluted by a factor of 40 (from 25 mL to 1000 mL), the concentration calculated from the titration must be multiplied by 40 to get the original concentration of NaOCl.
The %w/v of NaOCl is then calculated by using the molar mass of NaOCl to convert moles to grams, and dividing by the original volume of bleach sample, a common practice for consumer products, as seen in examples involving the labeling of liquid bleach concentrations. According to the information provided, a 100 g sample of bleach typically contains 7.4 g NaOCl, thereby indicating a 7.4% w/v concentration.
To report the %w/v NaOCl in the sample of bleach, we use the final calculated concentration of NaOCl, convert it to grams, and express it as a percentage of the volume of the bleach sample.