Answer to Question #156554 in General Chemistry for Julia

Question

A 300 ml water sample was taken from a stream in Mexico City and analyzed for dissolved oxygen. Using the winkler method, 58.88 ml of 2.42 x 10^-3 M sodium thiosulfate titrant was required. Calculate the dissolved oxygen in the water and express this as mg/L.

Answer 1

2S₂O$_3^{-2}$ + I$_3^-$ $\to$ 3I$^-$ + S₄O$_6^{-2}$

1000ml of 1N Na₂S₂O₃ = 8g dissolved oxygen

1N Na₂S₂O₃ = (1×2) M Na₂S₂O₃

1000ml of 2M Na₂S₂O₃ = 8g dissolved oxygen

2000 mmol Na₂S₂O₃ = 8g

1 m mol Na₂S₂O₃ = $\frac{8}{2000}$ g

55.88 ×2.42×10^-3 m mol Na₂S₂O₃ = $\frac{8×55.88×2.42×10^-³}{2000}$

= 0.0005409184 mg/L