A student does an experiment to determine the percentage by mass of sodium chlorate(I), NaClO, in a sample of bleach solution - AQA - A-Level Chemistry - Question 8 - 2020 - Paper 1

To calculate the mass of NaClO in the original bleach solution, we first determine the moles of sodium thiosulfate used.

1. Calculate moles of Na₂S₂O₃:

   Moles of Na₂S₂O₃ =
   \frac{33.50 , cm³}{1000} \times 0.00333 , mol , dm^{-3} = 0.000112 , mol

2. Use the stoichiometry from the equations:

   From the equations, we know that 2 moles of Na₂S₂O₃ react with 1 mole of I₃⁻. Thus: (0.000112 , mol , Na₂S₂O₃ \times \frac{1 , mol , I_3^-}{2 , mol , Na_2S_2O_3} = 0.000056 , mol , I_3^- )

3. Now, we find moles of NaClO:

   I₃⁻ is formed from ClO⁻ according to the reactions provided. Therefore, the moles of NaClO in the original solution: ( 0.000056 , mol , ClO^- \text{ from I3^-} )

4. Convert moles of NaClO to grams:

   The molar mass of NaClO is approximately 74.44 g/mol. Thus: (0.000056 , mol imes 74.44 , g/mol = 0.00417 , g)

5. Calculate the mass of bleach:

   Volume of original bleach = 10.0 cm³, density = 1.20 g/cm³: ( ext{Mass of bleach} = 10.0 , cm³ \times 1.20 \frac{g}{cm³} = 12.0 , g)

6. Calculate percentage by mass of NaClO in bleach:

   ( \text{Percentage by mass} = \frac{0.00417 , g}{12.0 , g} \times 100 = 0.0346 , % )

   Therefore, the percentage by mass of NaClO is approximately 0.035 % expressed to 3 significant figures = 0.0350%.