Good measurements are both accurate and precise. (These are not the same thing.) Accurate (correct)—How close the measurement is to the true value Precise (reproducible)—How close several measurements are to the same number (the measurement may be wrong) Measure your textbook.
Two common sources of error are human (the skill and knowledge of the person) and quality of the equipment Accepted value—the true or correct value based on reliable resources (you can look it up in a book) Experimental value—measured value determined by experiment in a lab Error=accepted value minus experimental value. Can be positive (exp. value less than actual value) Can be negative (exp. value greater than actual value)
Percent Error is the absolute value of the error divided by the accepted value and multiplied by 100 to get the percent. lAccepted value – experimental valuel x100 accepted value You will often calculate this value after a lab to determine your own % error.
Significant Figures are ones which are known to be reasonably reliable. This includes all digits which are known precisely plus one last digit that is estimated. The term significant does not mean certain. Table 6 Page 47 Table 6 Page 47
How many significant figures are in each of the following measurments? 1. 28. 6 g 2. 3440. cm 3. 910 m 4. 0.04604 L 5. 0.0067000 kg
Do Practice Problems 1-2 on Page 48 in your text.
Round to the same number of decimal places as the measurement in the calculation with the least number of decimal places. When working with whole numbers, the answer should be rounded so that the final significant digit is in the same place as the leftmost uncertain digit. (5400 + 365 = 5800) Example: Add 25.1 g and 2.03 g 27.1 g
Calculate and express in the correct number of sig figs. 5.44 m – 2.6103 m 2.83 m
Round to the number of significant figures in the least precise term used in the problem. Example: Calculate the density of an object that has a mass of 3.05 g and a volume of 8.47 mL. 0.360 g/mL
When using conversion factors to change one unit to another, they are usually exact measurements. For example, there are exactly 100 cm in a meter. This figure does not limit the degree of certainty in the answer. Example: Convert 4.608 m to centimeters. 460.8 cm
In scientific notation a number is written as the product of two numbers, a coefficient and a power of 10 Coefficient=a number greater than or equal to 1, but less than 10 (1.0 through 9.9) Exponent is a whole number that indicates how many times the coefficient must be multiplied by 10 (if positive exponent) or by 1/10 (if negative exponent)
Change a large number to scientific notation by moving the decimal to the left. The exponent will be a positive number. 65, 000 km 6.5 x 10 4 km
Change a small number to scientific notation by moving the decimal to the right. The exponent will be negative. 0.00012 mm 1.2 x 10 -4 mm
370.27 How many sigfigs? How can you write this number with the correct number of sigfigs? Use scientific notation.
Exponents must be equal. Then add or subtract the coefficients. Be sure to put final answer in correct scientific notation to the correct number of significant figures. 4.2 x 10 4 kg + 7.9 x 10 3 kg 5.0 x 10 4 kg Now do the problem on your calculator.
Multiply the coefficients and add the exponents algebraically. 5.23 x 10 6 μ m x 7.1 x 10 -2 μ m 3.7 x 10 5 μ m 2 Now do the problem on your calculator.
Divide the coefficients and subtract the denominator exponent from the numerator exponent. 5.44 x 10 7 g 8.1 x 10 4 mol 6.7 x 10 2 g/mol Now do the problem on your calculator.
Calculate the volume of a sample of aluminum that has a mass of 3.057 kg. The density of aluminum is 2.70 g/cm 3. 1.13 x 10 3 cm 3
Two quantities are directly proportional to each other if dividing them creates a constant value. Example—mass and volume of a substance (density is the constant They increase or decrease by the same factor. When one is doubled, so is the other. When one is halved, so is the other. Graph Page 55 Graph Page 55
Two quantities are inversely proportional if their product is a constant. When one is doubled the other is halved Graph Page 56-57 Graph Page 56-57
Make Flashcards of Common Elements Element Quiz 9/22
Sig Fig Worksheet Text Page 60 #35, 36, 37, 38, 43, 45 Element Flashcards—Do not need to be turned in. Density of Solids Lab Report Due on Friday 9/18 Print Element Survey Activity Read Ch 1.3 Pages 16-20