Unit 1 – Making Measurements Unit 1: Introduction to Chemistry and Measurements Credited to J. Wyatt GHS

Why Do We Measure? Measurements allow us to keep track of things, look at how things change over time, make comparisons, establish relationships between things, make predictions, and draw conclusions. They help us to understand the world around us!

What Do We Measure? Qualitative Measurements – descriptive measurements, not absolute Ex: Hot, cold, heavy, light, color, rough, smooth, smell, health Quantitative Measurements - numerical, units, defined, standardized Ex: time, length, density, speed, count – use SI Units!

Systeme Internationale (SI Units) Based on the metric system Helps for consistency among scientists Quantity Base Unit Symbol Time second s Length meter m Mass kilogram kg Temperature kelvin K Count/quantity mole mol Electric Current ampere A Luminous Intensity candela cd

Derived Units Not everything that can be measured can be measured with the seven base SI units Derived units to the rescue! Made of the seven base units Ex: Force = mass x acceleration Mass: measured in kg Acceleration: measured in m/s2 kg * m/s2 = Newton (N), unit of force

Why Use the Metric System? Makes conversions easy! English/Imperial Units have no standard conversion factor to make units larger or smaller Metric units are based on powers of 10 Fewer conversion factors to memorize! Can apply the conversion factor to any base unit, no matter what you are measuring.

Metric Prefixes You must memorize the ones that are starred! Remember…King Henry Died By Drinking Chocolate Milk! Prefix Symbol Factor Scientific Notation giga G 1 billion = 1 000 000 000 109 mega M 1 million = 1 000 000 106 *kilo k 1 thousand = 1 000 103 *hecto h 1 hundred = 100 102 *deca da ten = 10 101 *base unit none one = 1 100 = 1 *deci d 1 tenth = 0.1 10-1 *centi c 1 one hundredth = 0.01 10-2 *milli m 1 one thousandth = 0.001 10-3 micro µ 1 one millionth =0.000 001 10-6 nano n 1 one billionth = 0.000 000 001 10-9 Make the base unit BIGGER Make the base unit smaller

Accuracy and Precision Is there a difference? YES!!! Accuracy is getting close to your target; close to the accepted value of the measurement Precision is getting the same answer over and over again, regardless of how correct (accurate) the measurement is Good measurements are BOTH!

Accuracy and Precision Examples If the true density of the substance is 1.59 g/cm3, which student collected the most accurate data? Which student was the most precise? Density Data Collected by Three Different Students Student A Student B Student C Trial 1 1.54 g/cm3 1.40 g/cm3 1.70 g/cm3 Trial 2 1.60 g/cm3 1.68 g/cm3 1.69 g/cm3 Trial 3 1.57 g/cm3 1.45 g/cm3 1.71 g/cm3 Average 1.51 g/cm3

Accuracy and Precision Examples Four students each took 3 temperature readings of a sample of water. The actual temperature of the water was 80.0°C. Which student’s measurements were both accurate and precise? Student Measurements of Temperature Reading 1 (°C) Reading 2 (°C) Reading 3 (°C) Student 1 78.6 78.5 78.7 Student 2 82.4 80.0 81.4 Student 3 80.8 78.9 81.8 Student 4 80.1 79.9

If you had to measure 15.0 mL of a liquid, which would you choose? Making Measurements If you had to measure 15.0 mL of a liquid, which would you choose?

Estimating Measurements Must always read all of the certain numbers plus one estimated digit The pencil is 18.73 cm long The decimal place of the estimated digit is determined by how the instrument is calibrated The estimated digit is in the 0.01 (one hundredths) place This instrument is precise to the 0.01 place Which of the rulers to the right would allow you to be the most accurate and precise in your measurement?

Significant Figures All of the numbers that we are certain about + estimated digit are significant to the measurement Called significant figures (sig figs) or significant digits (sig digs) When numbers are converted into scientific notation, the number of sig figs must be the same as in the original measurement Only measurements have sig figs! Definitions and counting numbers have infinite sig figs Ex: 12 cats Ex: 12 inches = 1 foot

Determining Significant Figures Any number that is not a zero is always significant Zeros may or may not be! Trailing zeros are only significant if a decimal point is present A zero in between two non-zero numbers is significant Zeros that act as place holders are not significant

Determining Significant Figures – Made Easy! The rules for sig figs can be simplified by applying the Atlantic/Pacific rule Ask yourself – Is there a decimal? Yes! – Pacific (present) – start from the left and count all numbers from the first non zero number NO! – Atlantic (absent) – start from the right and count all numbers from the first non zero number

Uncertainty in Measurements Sig figs help you to determine where the uncertainty in your measurements lies The last significant figure is the estimated one – this is the uncertain digit All of these scales measure mass, but their uncertainty is different. Uncertainty +/- 0.1 Uncertainty +/- 0.01 Uncertainty +/- 0.001 Uncertainty +/- 0.0001 Think about how the mass of the same object would look on these different scales.

Uncertainty in Measurements - Range Since the scales are rounding to different places, the real value lies within a range. The uncertainty defines that range. If the object on the first scale reads 12.3 g, then the true value should be between 12.2g and 12.4 g (12.3 +/- 0.1 g) On the second scale, the uncertainty is +/- 0.01 g, so a possible measurement might be 12.35 g, and the true value should be between 12.34g and 12.36 g (12.35 +/- 0.01g) On the third scale, the uncertainty is +/- 0.001 g, so a possible measurement might be 12.348 g, and the true value should be between 12.347 and 12.349 g (12.348 +/- 0.001g) What is a possible value for the same object if it were placed on the last scale?

Explain This! Using what you know about measurements, how could this possibly be true?

Calculating With Measurements Oftentimes we have to add, subtract, multiply, or divide with measurements. We must take uncertainty into account when we do this! Your final answer cannot be more certain than your least certain measurement. 56.0 g / 13 cm3 = 4.307692307692308 g/cm3 …..etc. Where do you cut off the calculator vomit? Sig Figs to the rescue!

Adding and Subtracting Measurements Your final answer must have the same number of decimal places as your measurement with the fewest decimal places when you add and subtract measurements Add normally then round answer to the appropriate place

Multiplying and Dividing Measurements Your final answer must have the same number of significant figures as your measurement with the least amount of significant figures Calculate normally then round final answer to the proper number of sig figs

Rounding is Serious Stuff! Twenty-eight Americans were killed on February 25, 1991 when an Iraqi Scud hit the Army barracks in Dhahran, Saudi Arabia. The Patriot defense system had failed to track and intercept the Scud