1 Accuracy and Precision Notes Chemistry 1

2 Uncertainty in Measurements There is no such thing as a perfect measurement! All measurements have a degree of uncertainty because –Instruments have flaws –Measuring always requires estimation on the last digit But we want measurements to be as accurate and precise as possible

3 Accuracy The CORRECTNESS of a measurement How close a measurement’s value is to the accepted (actual or true) value Can be described for a single measurement or for the average of a group of measurements

4 Accuracy is evaluated by –comparing the experimental value to the accepted value Experimental value is the value that is measured in the lab Accepted value is the true or correct value based on reliable references –calculating the % error for the measurement %error = experimental value – accepted value x 100 accepted value –The closer the % error is to 0, the better the accuracy

Notice that if your % error is negative, that means your experimental value was too low If your %error is positive, that means your experimental value was too high 5

6 Calculating % error- examples A student measured the boiling point of a liquid during a lab experiment and obtained a value of 99.1 ◦ C. The accepted value for the boiling point of the liquid is ◦ C. What is the percent error of the student’s measurement? Would you classify this student’s measurement as accurate or inaccurate?

7 Lab group #1 measured the mass of a piece of copper and obtained a value of 10.5 grams. The accepted value for the mass of this copper is 9.1 grams. Calculate the %error of this group’s measurement. Was this lab group’s measurement accurate?

8 Precision How REPRODUCIBLE a measurement is How close in value a group of measurements are to one another Can’t be described for a single measurement; only for a group of measurements Also depends on the smallest divisions (or increments) on the scale of the measuring instruments –The smaller the increments, the more precise the measurements will be

9 We can evaluate the precision of a group of measurements by looking at the range between the smallest and largest measurement in the group –The smaller the range, the better the precision

Lab group 1 made measurements and calculated the density of a metal in lab. For trial #1, their value was 3.45g/cm 3. For trial #2, their value was 3.78g/cm 3. Lab group #2 had values of 3.12g/cm 3 and 3.15g/cm 3. Which lab group’s measurements were more precise? 10

11 Examples- Accuracy and Precision gfigs/accuracy_and_precision.htmhttp:// gfigs/accuracy_and_precision.htm sigfigs/B-Acc-Prec-Unc.htmlhttp://scidiv.bcc.ctc.edu/Physics/Measure& sigfigs/B-Acc-Prec-Unc.html