Measurement in Chemistry

Reporting Measurements Report what is known with certainty Add ONE digit of uncertainty (estimation) By adding additional numbers to a measurement – you do not make it more precise. The instrument determines how precise it can make a measurement. Remember, you can only add ONE digit of uncertainty to a measurement.

Practice Measuring cm 1 2 3 4 5 4.5 cm cm 1 2 3 4 5 4.54 cm cm 1 2 3 4 1 2 3 4 5 4.5 cm cm 1 2 3 4 5 4.54 cm PRACTICE MEASURING Estimate one digit of uncertainty. a) 4.5 cm b) * 4.55 cm c) 3.0 cm *4.550 cm is INCORRECT while 4.52 cm or 4.58 cm are CORRECT (although the estimate is poor) By adding additional numbers to a measurement – you do not make it more precise. The instrument determines how precise it can make a measurement. Remember, you can only add ONE digit of uncertainty to a measurement. In applying the rules for significant figures, many students lose sight of the fact that the concept of significant figures comes from estimations in measurement. The last digit in a measurement is an estimation. How could the measurement be affected by the use of several different rulers to measure the red wire? (Different rulers could yield different readings depending on their precision.) Why is it important to use the same measuring instrument throughout an experiment? (Using the same instrument reduces the discrepancies due to manufacturing defects.) cm 1 2 3 4 5 3.0 cm

Significant Figures What is the difference between a measurement of 27 mL and 27.3 mL? 27.34 mL?

Significant Figures What is the difference between a measurement of 20 mL and 20. mL? 20.0 mL? 20.00 mL?

Significant Figures What are they? All “certain” digits and ONE “uncertain” digit in a scientific measurement or calculation. What do we mean by “certain” and “uncertain”?

Significant Figures Why do we use them? To account for the limits of ourselves. To account for the limits of our instruments. Basically, we use sig. figs. to keep from lying about the precision of our instruments.

Significant Figures How do I use sig. figs. when measuring? If the instrument is digital, record every digit (even zeroes). If the instrument is lined (rulers, graduated cylinders, etc.), estimate one digit beyond the markings.

Significant Figures How do I tell if a digit in a number is significant? All nonzero digits are significant. All “captured” zeroes are significant. “Leading” zeroes are never significant. “Trailing” zeroes are significant only if the number has a decimal point in it.

Significant Figures “Captured” zeroes are zeroes with nonzero digits both to the right and the left. 700.85 has two captured zeroes .9302 has one captured zero 10.6005 has three captured zeroes Again, captured zeroes ARE significant

Significant Figures “Leading” zeroes are zeroes with no nonzero digit to their left. 0.85 has one leading zero .000932 has three leading zeroes 0.025 has two leading zeroes Again, leading zeroes ARE NOT significant

Significant Figures “Trailing” zeroes are zeroes with no nonzero digit to their right. 8500 has two trailing zeroes 93.2000 has three trailing zeroes 25.0 has one trailing zero Trailing zeroes are significant IF and ONLY IF there is a decimal point written in the number.

Significant Figures How many sig. figs. do these numbers contain? 3.582 0.503 4600 520. 0.060050 1800.03 .1000 3250 .0036 60