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Homework Help: Rate of reaction

  1. Apr 9, 2007 #1
    I am currently doing an experiment.

    The experiment is to investigate the dependence of reaction rate on conc. for the reaction between sodum thiosulphate reacts with dilute acids

    By varying concentrations of thiosulphate and H+ respectively in 2 experiments.We can easily find that speed of reaction is proportional to conc. of reaction.

    the thing I want to ask is

    Drops of thiosulphate 1 2 3 4 5 6 7
    Drops of water 6 5 4 3 2 1 0

    Drops of H+ 7 7 7 7 7 7 7
    Drops of water 2 2 2 2 2 2 2

    what do we need to make the total volume constant for the reacions?
    is it because to standadize the experimentally condition , each time only 20cm3 thiosulphate solution is used.Addition of water enables different conc. of thiosulphate all at 20 cm3????
    I am really frustrated .

    Last edited: Apr 9, 2007
  2. jcsd
  3. Apr 10, 2007 #2
    Can anyone help me?
  4. Apr 10, 2007 #3


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    Measure the density of the resulting solution and record the total mass of solution to correct for the effect of dilution. Plot your data using the corrected concentrations. Since you are using very small reaction sizes (drops?) you will have to use very small calibrated volumetric pipets and an accurate analytical scale.

    You may find that you can assume that the density of the resulting solutions are a weighted average of the densities of the original thiosulfate solution and the acid solution. This will require you to accurately weigh the amounts of solution you add (weigh a drop of thiosulfate ten times and use the average).
  5. Apr 10, 2007 #4
    Since I carry out a microscale experiment , So I need to use "DROPS".
    So the total volume should kept constant because I can accurately measure the densisities of cpds? But I am not fully understand it
    The experiment that I carried out is to find the relationship between conc. of reactant and rate of reaction.So how does it relate to teh total volume of solution?
  6. Apr 10, 2007 #5


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    If you have a given concentration of thiosulfate (or acid) in a drop, you will add a certain amount of the reagent to the solution dropwise. If you know the total mass of the solution after your addition and its density you can calculate the total volume of solution:

    Volume = Mass/density

    This will give you the updated volume of the solution and allow you to correct the concentration due to dilution.

    If you have a solution containing several different species from several different solutions (each with unique densities) you can calculate the density of the final solution using a weighted average.


    Sample of solution that has 6 drops of solution 1, 8 drops of solution 2 and 4 drops of solution 3.

    Solution 1 density 1.0000 g/mL average weight of drop is 0.087 grams
    Solution 2 density 1.0360 g/mL average weight of drop is 0.092 grams
    Solution 3 density 1.0041 g/mL average weight of drop is 0.089 grams

    Total drops in solution is 18 drops that weighs 1.614 grams (6 drops * 0.087 g/drop + 8 drops * 0.092 g/drop + 4 drops * 0.089 g/drop). Alternatively, you could perform the addition to your cuvette while it is on an analytical scale. What is important is the total weight of solution and the weights of the added solutions. The calculated density will be:

    density = (weight of added Solution 1/total weight)*density of Solution 1 +
    (weight of added Solution 2/total weight)*density of Solution 2+
    (weight of added Solution 3/total weight)*density of Solution 3

    Substituting yields:

    density = (0.522g/1.614g)*1.000 + (0.736g/1.614g)*1.036 + (.356g/1.614g)*1.0041

    density = 0.3234 + 0.4724 + 0.2215 = 1.017 g/mL

    volume of solution is therefore:

    volume = 1.614g/1.017g/mL = 1.587 mL

    This is now Solution 4, about which you know the volume and the amount of each reagent added. Addition of a few drops of any of the other solutions to it will yield a new solution that can be analyzed in a similar fashion.

    You don't need to keep the volume constant... just keep track of the volume and update the concentrations after each addition. The calculations will be lengthy and you should be very methodical. Tabulate your data at each datapoint.
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