1. Measuring Significantly Count significant digits because significant digits count!

2. Review Accuracy: – How close the value is to the correct or accepted value – Is the measuring instrument properly calibrated? Precision: – How exact or repeatable is the value – What is the quality of the measuring device and measurement?

3. How do we judge accuracy and precision? Accuracy? – % error Precision? – Significant Digits…sig digs – Also called significant figures…sig figs

4. Both rulers are calibrated properly for accuracy. Which is the most precise? How do we communicate that? 1.5 cm (which= 15 mm) 1.50 cm (which = 15.0 mm)

5. What does “measuring significantly” mean? It means that you are using the measuring instrument to the highest level of precision that it offers. You are expressing that level of precision by the number of significant figures that you use to report the measured value.

6. What is a significant figure? Significant figures in a measurement consist of all the digits known with certainty plus one final digit, which is somewhat uncertain or is estimated. The term significant does not mean certain.

7. Why is it important? The only way that another individual will know the precision of your quantitative laboratory work is by the significant digits that you report. Remember the story of the very expensive steel cube that was made to measure 1.000 cubic centimeters when 1.0 cubic centimeters would have sufficed! I’m worth my weight in gold!!!

8. How do you measure significantly? Note the smallest units that are marked in any way on the device (not just the marks with labeled numbers). Carefully read the measurement. – Read with certainty to the smallest unit marked. – Estimate one place beyond the smallest unit marked. Note the value with all significant digits, even if they are zeros. Show the units.

9. ATLANTIC PACIFIC RULE Count in the correct direction from 1st non-zero digit and keep counting!

10. A trick… To determine if zeroes are significant parts of the measured value or placeholders – Picture your value on a map of the USA, with Atlantic to the right and Pacific to the left – If decimal point is Absent count digits from the Atlantic side, starting with the first non-zero digit – If decimal point is Present count digits from the Pacific side, starting with the first non-zero digit – Keep counting once you start

11. You must use significant measurements in calculations! Always write the formula you are using. Plug in the values using the correct number of significant digits with units. Do the math, combine or cancel units. When doing calculations with measured values, report your answer as it is limited by the least significant measurement. (use the rules for calculating with sigdigs.

12. EXAMPLE: D = mass /volume = m /v = g / cubic cm OR g / mL Wood= 34.85 g / 66.8 cubic cm = 0.521 g / cu cm Metal = 18.37 g / 7.2 mL = 2.55 = 2.6 g /mL Use same # of sigdigs for % error calculations.