Chemistry Measurement Notes Chapter 3: Significant Figures, Scientific Notation, Metric Conversions

Measurement Units: Without units, data has no meaning…. Mass = gram (g) Volume = cubic centimeter (cm3), Liter (L) Length = meter (m) Temperature= degrees Celsius (°C) or Kelvin (K) Density= mass/ volume (g/mL or g/cm3) Heat= Joules (J) or calories (cal) Without units, data has no meaning….

Uncertainty When reading a measurement from a device, always estimate one extra digit. This device measures in increments of 1 degree. We know that there is at least 3 degrees, we know there is not more than 4, we have to estimate to the tenths place I’d say 3.8 degrees where .8 is the uncertain digit.

Accuracy vs. Precision Accuracy- how close you are to the accepted value Precision- ability to reproduce results, getting the same answer over and over Example- a substance has a density of 3.00g/mL. In the lab, you find the density to be: Trial 1- 6.5 g/mL, Trial 2- 6.55 g/mL, Trial 3- 6.53 g/mL. Therefore, your data is NOT accurate, but is precise.

Metric Conversions 3 Metric Prefixes you must know: 1 kilo = 1000 base Ex: 1 kilogram= 1000g 100 centi = 1 base Ex: 100 centimeters=1meter 1000 milli = 1 base Ex: 1000 milliliters = 1 liter. You must also know this: 1 mL= 1 cm3 Setting up conversion factors: Convert 45 mm to km Convert 5 m/min to km/hr.

Significant Figures (Sig Figs) All nonzero digits are significant. Ex: 3.134 has 4 sig figs Zeros to the right of the decimal are significant, but only if there’s a sig fig before. Ex: 2.00 has 3 sig figs. 0.0024 has 2 sig figs. Zeros to the left of the decimal are NOT significant. Ex: 0.15 has 2 sig figs. Zeros in-between digits are significant. Ex: 0.2005 has 4 sig figs.

Sig Figs Continued When adding/subtracting with sig figs, you must report to the least number of decimal places. used. Ex: 12.52m + 349.0m + 8.24m = 369.76m but you can only use 1 decimal so 369.8m For multiplying & dividing. Only report to fewest sig figs used. Ex: 2.40 * 1.2 = 2.88 but can only use 2 sig figs so answer is 2.9 (rounding rules apply).

Scientific Notation Scientific Notation is used to simplify very large or very small numbers. The only digits used in S.N. are sig figs. Ex: 1,200,000,000 is 1.2x109 Ex: 0.000012 is 1.2x10-5 There can only be 1 non-zero digit in front of the decimal.

Calculating with S.N. When you multiply, multiply the numbers and add the exponents, Ex: 2.0x102 x 4.0x103= 8.0x105 When you divide, divide the numbers and subtract the exponents. Ex: 4.6x106 ÷ 2.2x104=2.3x102 For addition and subtraction, the numbers must have the same power of ten. You add/subtract the numbers and keep the same power of ten. Ex: 4.0x103-3.0x103 = 1.0x103

Celsius to Kelvin 0 Degrees on the Kelvin Scale is considered absolute zero- the temperature where there is no molecular motion (theoretical). K = °C + 273 C = K -273

Error & Uncertainty in Measurement Plus/Minus Notation Used to show how much uncertainty there is in a measurement. Ex: a leaf is 7cm ±0.5 cm. This means the leaf could be 7.4 or it could be 6.5. Percent Error % Error = absolute error / true value x100 Absolute error is the difference between experimental and true value. Ex: You found the density of water to be 1.7 g/ml. True value is 1.0 g/ml. % error is .7/1.0= 70%