Question

Post-Lab Questions: Answer. 1. What should you examine to determine the precision of your results? Explain. 2. What should you examine to determine the accuracy of your results? Explain. 3. If the standardization of the NaOH in Experiment 7 was not performed correctly and the experimental normality was too low, how would that affect the experimental equivalent weight being determined in this experiment? Would it be too high or too low? Explain. 4. You were allowed to weigh out between 0.35 g and 0.40 g of the unknown sample. If you used 0.35 g for Trial #1 and 0.40 g for Trial #2, would the experimental equivalent weight for Trial 2 be higher, lower or the same as that of Trial #1? Explain. ACID-BASE TITRATION Determination of the equivalent weight of an unknown acid.

Answer 1

Let's take an example. Suppose you want to find the mass of a gold ring that you would like to sell to a friend. You do not want to jeopardize your friendship, so you want to get an accurate mass of the ring in order to charge a fair market price. You estimate the mass to be between 10 and 20 grams from how heavy it feels in your hand, but this is not a very precise estimate. After some searching, you find an electronic balance that gives a mass reading of 17.43 grams. While this measurement is much more precise than the original estimate, how do you know that it is accurate, and how confident are you that this measurement represents the true value of the ring's mass? Since the digital display of the balance is limited to 2 decimal places, you could report the mass as

m = 17.43 ± 0.01 g.

Suppose you use the same electronic balance and obtain several more readings: 17.46 g, 17.42 g, 17.44 g, so that the average mass appears to be in the range of

17.44 ± 0.02 g.

By now you may feel confident that you know the mass of this ring to the nearest hundredth of a gram, but how do you know that the true value definitely lies between 17.43 g and 17.45 g? Since you want to be honest, you decide to use another balance that gives a reading of 17.22 g. This value is clearly below the range of values found on the first balance, and under normal circumstances, you might not care, but you want to be fair to your friend. So what do you do now? The answer lies in knowing something about the accuracy of each instrument.