CHEMISTRY August 2, 2012

Place Homework on your desk (2 mins) CHEMISTRY BRAIN SAMPLER August 2, 2012 Place Homework on your desk (2 mins) 1. How many sig figs are in 0.00236 2.030 50.00 102,000 2. What is the SI unit for mass? temperature? 4. Refer to the diagram on the board. Report the measurement? (include an appropriate unit)

Agenda Measurement Lab and Post Lab Brain Teaser Demo Grade Homework Measurement Worksheet 2 Measurement Lab and Post Lab Notes: Accuracy and Precision Significant Figures Calculations Homework Accuracy and Precision Worksheet Post Lab Questions

Accuracy and Precision

Data Terms Quantitative Measurements Examples Give results in a definite form, usually values 24L, 10 cm, 14 ºC 5

Data Terms Qualitative Measurements Examples Give results in a descriptive, non-numeric form. The beaker was warm. The density was greater than that of water. 6

Data Terms Accuracy Examples How close a measurement comes to the actual value of whatever is being measured Water freezes at 0º C, and boils at 100º C. How close is the measurement to the values. 7

Data Terms Precision Examples Reproducibility of the measurement 9 out of 10 lab groups report the temperature of boiling water to be 95º C. A basketball player shoots 20 free throws, 18 of which bounce off the right side of the rim. 8

Accuracy vs. Precision Target Practice Accurate Precise Accurate & Precise 9

An archery target illustrates the difference between accuracy and precision. 10

Scale Reading and Uncertainty Uncertainty: Limit of precision of the reading (based on ability to guess the final digit). Existed in measured quantities versus counted quantities Addition and substraction(no. of sig. figs) Division and Multiplication (no. of sig. figs) 11

Percent error Theoretical – Experimental x 100 = % error Theoretical 12

Significant Figures in Calculations

Significant Figures in Calculations What are Significant Digits? Examples Triple Beam Balance Graduated Cylinder All the certain digits plus the estimated digit in a measurement. How many decimal places can we count

Significant Figures in Calculations Exact Numbers Examples Infinite # of sig figs Do not affect the number of significant digits in the final answer. They are not measurements!! 1000m = 1 km 12 in = 1 foot

Significant Figures in Calculations Multiplication and Division Example The number with the smallest number of significant digits determines how many significant digits are allowed in the final answer. Volume of a box L x W x H (3.05m)(2.10m)(0.75m) 2 sig figs 4.8m3

Significant Figures in Calculations Example Density of a penny M = 2.53g V = 0.3mL D = M / V # significant figures allowed D = 8g/mL

Significant Figures in Calculations Addition and Subtraction Example The number of significant digits depends on the number with the largest uncertainty. (you may be using different scales) Shoes 951.0 g Clothes 1407 g Ring 23.911 g Glasses 158.18 g Total 2540. g

Significant Figures in Calculations Example What is the mass of a penny if, the weighing paper alone has a mass 0.67 g and weighing paper plus the penny has a mass of 3.2 g. 3.2 g -0.67 g 2.5 g

Significant Figures in Calculations Remember A calculated number can only be as precise as the least precise measurement in the calculation.

Practice Calculate each of the following to the correct number of significant figures. Include units on your answer. (25 g/mol)(4.0 mol) = (3.48 in)(1.28 in)(0.010 in) = 2.06 cm + 1.8 cm + 0.004 cm = If the mass of a lead cube is 176.91 g and it measures 2.51cm x 2.49 cm x 2.49 cm, what is the density of lead?

Practice Calculate each of the following to the correct number of significant figures. Include units on your answer. (25 g/mol)(4.0 mol) =1.0 x 102 (3.48 in)(1.28 in)(0.010 in) = .045 in3 2.06 cm + 1.8 cm + 0.004 cm = 3.9 cm If the mass of a lead cube is 176.91 g and it measures 2.51cm x 2.49 cm x 2.49 cm, what is the density of lead? 11.3 g/cm3 Period 1 Stop Here 22

Rally rows Sig figs in Calculations 12 cm + 0.031cm + 7.969 cm = (41.025 g - 23.38g) ÷ 8.01 mL= 17.3 cm x 6.2 cm + 3.28 cm2 = 109.3758 m2  45.813 m = What is the mass of Salt (NaCl) if the sodium has a mass of 22.99 g and the Cl a mass of 35.5g?