Accuracy and Precision Notes Chemistry 1 1 Uncertainty

Accuracy and Precision Notes Chemistry 1 1

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 2

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 3

• 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 – The closer the % error is to 0, the better the accuracy 4

• 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

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 102. 5 ◦ C. What is the percent error of the student’s measurement? Would you classify this student’s measurement as accurate or inaccurate? 6

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? 7

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 8

• 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 9

• Lab group 1 made measurements and calculated the density of a metal in lab. For trial #1, their value was 3. 45 g/cm 3. For trial #2, their value was 3. 78 g/cm 3. • Lab group #2 had values of 3. 12 g/cm 3 and 3. 15 g/cm 3. • Which lab group’s measurements were more precise? 10

Examples- Accuracy and Precision • http: //www. carlton. srsd 119. ca/chemical/Si gfigs/accuracy_and_precision. htm • http: //scidiv. bcc. ctc. edu/Physics/Measure& sigfigs/B-Acc-Prec-Unc. html 11