General Chemistry Lab - Accuracy, Precision, and Significant Figures PDF

General Chemistry Lab FIRST LECTURE: Accuracy – Precision – Significant Figures Instructor: Mrs. Nahid Chehade  Precision and Accuracy Precision: refers to how close measurements of the same item are to each other. Accuracy: refers to how close a me...

General Chemistry Lab FIRST LECTURE: Accuracy – Precision – Significant Figures Instructor: Mrs. Nahid Chehade  Precision and Accuracy Precision: refers to how close measurements of the same item are to each other. Accuracy: refers to how close a measurement is to the true or accepted value Note Precision is independent of accuracy 2 Precision is independent of accuracy. That means it is possible to be very precise but not very accurate, and it is also possible to be accurate without being precise. The best quality scientific observations are both accurate and precise. 3 Examples: Accurate and precise: If a weather thermometer reads 25oC outside and it is really 25oC, the thermometer is accurate. If the thermometer consistently registers the exact temperature for several days in a row, the thermometer is also precise. Precise, but not accurate: A refrigerator thermometer is read ten times and registers degrees Celsius as: 39.1, 39.4, 39.1, 39.2, 39.1, 39.2, 39.1, 39.1, 39.4, and 39.1. However, the real temperature inside the refrigerator is 37◦ C. The thermometer isn’t accurate (it’s almost two degrees off the true value), but as the numbers are all close to 39.2, it is precise. 4 Significant Figures: In chemistry, Significant figures are the digits of value which carry meaning towards the resolution of the measurement. They are also called significant figures in chemistry. Rules for Significant Figures: Rule 1: Non-zero digits are always significant. Rule 2 : Captive zeros, zeros between two significant digits are significant. Rule 3: Trailing zeros, zeros at the end of a number in the decimal portion ONLY are significant Rule4: Leading zeros, zeros that precede a number ARE NOT counted significant ❑ The Below Table Shows examples on counting significant figures: 5  Count the number of significant figures for each number Zeros in between non- 101.001 6 zero digits are 41003 5 significant. Zeros at the end of the 500 1 answer 13000 2 when no decimal point 140 2 is specified are not significant. Zeros at the end of the 500. 3 answer when a decimal 5.0 2 point is specified are 2.000 4 significant. 8.20000 6 6 Significant Figures in Mathematical Operations: Multiplication and Division The following rule applies for multiplication and division: The LEAST number of significant figures in any number of the problem determines the number of significant figures in the answer. For addition and subtraction, look at the decimal portion (i.e., to the right of the decimal point) of the numbers ONLY. The answer is given the same number of decimal places as the term with the fewest decimal places. 7 Examples: Calculate the answers to the appropriate number of significant figures. 32.567 + 135.0 +1.4567 = 169.0237 = 169.0 658.0 + 23.5478 + 1345.29 =2026.8378 = 2026.8 23.7 x 3.80 = 90.06 = 90.1 81.04 x 0.010 =0.8104 = 0.81 1.678 / 0.42 = 3.9952 = 4.0 4278 / 1.006 = 4252.48509 = 4252 8