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10.0 cm3 of iodine solution was titrated with 0.100 M sodium thiosulfate solution . The titres were 12.30 cm3, 11.80 cm3 and 11.90 cm3.
What indicator is used and what is the colour change at the end-point?
What is the concentration of the iodine solution in g dm-3?
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Starch indicator is used and the colour change at the end-point is blue-black to colourless. |
Average titre (ignoring 12.30 cm3) = (11.80 + 11.90)/2 = 11.85 cm3
Number of moles of Na2S2O3 = 11.85/1000 × 0.100
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2Na2S2O3(aq) + I2(aq) ⇒ Na2S4O6(aq) + 2NaI(aq) |
I2 º2Na2S2O3
\ number of moles of I2 in 10.0 cm3 = ½ × 11.85/1000 × 0.100
Volume of iodine solution = 10.0/1000 = 0.0100 dm3
\ concentration of I2 = (½ × 11.85/1000 × 0.100)/0.0100 = 0.0593 mol dm-3
I2 = 254 [don't forget that there are two moles of iodine in I2]
\ concentration of I2 = 0.059 × 254 = 15.0 g dm-3
