Uncertainty
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Summary

After completing this module you should more fully understand the importance of making proper use of significant figures in your calculations. You should also appreciate that the uncertainty of your experimental work is limited by the measurement with the greatest uncertainty. When planning an experiment, think carefully about how you will make use of your measurements. For example, suppose you are given the following directions:

Add 50 mL of water to a 250-mL volumetric flask. Transfer about 0.1 g of KCl to the volumetric flask, swirl until the solid dissolves and then dilute to volume. Calculate the exact concentration of KCl in your solution.

To calculate the concentration of KCl you need to know the mass of KCl. You will want, therefore, to measure this using a balance that weighs samples to the nearest ±0.001 g or ±0.0001 g. If available, the later choice is best as it provides a smaller uncertainty. Making a decision to replace the volumetric flask with an Erlenmeyer flask is a mistake since the greater uncertainty in an Erlenmeyer flask's volume increases your uncertainty in the concentration of KCl. On the other hand, the 50 mL of water does not appear in your calculation; for this reason it is not necessary to measure accurately this volume and a graduated cylinder will suffice.

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