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This is repeated for a range of concentrations of the substance you are interested in.You would need to cover a reasonably wide range of concentrations, taking perhaps 5 or so different concentrations varying from the original one down to half of it or less.That's because in a first order reaction, the rate is proportional to the concentration. It might be second order - but it could equally well have some sort of fractional order like 1.5 or 1.78.
If you were looking at the effect of the concentration of hydrogen peroxide on the rate, then you would have to change its concentration, but keep everything else constant.
The temperature would have to be kept constant, so would the total volume of the solution and the mass of manganese(IV) oxide.
That is only a reasonable approximation if you are considering a very early stage in the reaction.
The further into the reaction you go, the more the graph will start to curve.
Measuring the slope of a straight line is very easy. Now suppose you did the experiment again with a different (lower) concentration of the reagent.
Again, we will measure the time taken for the same volume of gas to be given off, and so we are still just looking at the very beginning of the reaction: The initial rates (in terms of volume of gas produced per second) are: of gas, you just collected the gas up to a mark which you had made on the side of a test tube. If you are simply wanting to compare initial rates, then it doesn't matter.The maths of this might not be familiar to you, but you may find that you are asked to do this as a part of a practical exam or practical exercise.If it is an exam, you would probably be given help as to how to go about it.You can then plot 1/t as a measure of rate against the varying concentrations of the reactant you are investigating.If the reaction is first order with respect to that substance, then you would get a straight line. I wouldn't really recommend that you try to read it all in one go.Or it could be the time taken for a small measurable amount of precipitate to be formed.You could also use a special flask with a divided bottom, with the catalyst in one side, and the hydrogen peroxide solution in the other. If you use a 10 cm measuring cylinder, initially full of water, you can reasonably accurately record the time taken to collect a small fixed volume of gas.You could, of course, use a small gas syringe instead.Since this is the part of the reaction you are most interested in, introducing errors here would be stupid!You have to find a way of adding the catalyst to the hydrogen peroxide solution without changing the volume of gas collected.