1. Look at the procedure below. At the time of mixing, what are the nominal conc
ID: 961425 • Letter: 1
Question
1. Look at the procedure below. At the time of mixing, what are the nominal concentrations of H2O2 and S2O32- in the reaction mixture prepared in step 1 of the procedure? Assume that the stock concentrations of H2O2 and Na2S2O3 are 0.2 and 0.02 mol/L, respectively, and that the volumes specified in the procedure were buretted exactly.
2. How much of each of these two species has reacted by the time a blue colour appears exactly 3 min 10s after mixing?
3. Calculate the rate of reaction for this hypothetical case.
Theoretical Procedure:
1. Burette 10 mL of sodium thiosulphate solution into a clean 250 mL Erlenmeyer flask. Next burrette 40 mL of buffered iodide solution (contains the starch indicator). In a clean beaker, burette 10 mL of stock H2O2(aq) and 20 mL of distilled water. All at once, pour the diluted H2O2 into the flask and start timing. Swirl the contents and measure the time interval (±1s) between mixing and the appearance of colour.
Explanation / Answer
Q.1: Total volume at the time of mixing, Vt = 10 mL + 40 mL + 10 mL + 20 mL = 80 mL = 0.080 L
Moles of H2O2 added = MxV = (0.2 mol/L) x 10 mL x (1 L / 1000 mL) = 0.002 mol
Hence concentration of H2O2 at the time of mixing = moles of H2O2 / Vt
= 0.002 mol / 0.080 L = 0.025 M (answer)
Moles of Na2S2O3 added = MxV = (0.02 mol/L) x 10 mL x (1 L / 1000 mL) = 0.0002 mol
Hence concentration of Na2S2O3 at the time of mixing = 0.0002 mol / 0.080 L = 0.0025 M (answer)
Q.2: The balanced reaction is
H2O2 + 2I + 2H+ --------- > I2 + 2H2O
2S2O32 + I2 -------- > S4O62 + 2I
Dark blue color appeard when Na2S2O3 is completely consumed during the reaction.
Hence moles fo Na2S2O3 reacted = 0.0002 mol (answeR)
In the above balanced reaction 1 mol of H2O2 reacts with 2 mol of Na2S2O3.
Hence 0.0002 mol of Na2S2O3 that will react with the moles of H2O2
= 0.0002 mol Na2S2O3 x ( 1 mol H2O2 / 2 mol Na2S2O3) = 0.0001 mol H2O2 (answer)
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.