Test 2: Standards of Measurement Chemistry Test 2: Standards of Measurement

What is a STANDARD? It is an exact quantity that people agree to use for comparison

Measurement A measurement must have a number and a unit. (No Naked Numbers!)

International System of Units SI is an improved version of the metric system and is used by scientists worldwide. It is based on multiples of ten and uses prefixes to indicate a specific multiple. Copy Table 2-2 “Prefixes Used with SI Units” on p.26 of your textbook. Copy Table 2-1 “SI Base Units” on p.26 of your textbook.

Converting Between SI Units You will need a conversion factor (a ratio that is equal to one) for dimensional analysis. Example: We use the processes of dimensional analysis and moving the decimal to convert between units.

Length Is a measure of the distance between two points Is measured using a unit appropriate for the distance between the two points SI base unit is the meter

Volume Is the amount of space an object contains The units are: liters, cubic meters (centimeters, etc.) The volume for a solid rectangle can be measured in cubic centimeters by using the formula V=L x W x H The volume of liquids is most commonly measured in liters and milliliters. A liquid has no sides to measure so the measurement is actually the capacity of the container holding the liquid. 1 mL = 1 cm3 To calculate the volume of an irregular solid, we can submerge it in a known volume of liquid and read the new volume level. The volume of the object is equal to the change in volume of the liquid. Volume is a derived unit.

Mass Is the measure of matter in an object The most commonly used units for mass are grams and kilograms. Remember the difference between mass and weight!

Density Is the mass per unit volume of a material; D = m/v It is a derived unit (a unit obtained by combining different SI units) Density can be used to identify a substance. The most common units for density are g/mL and g/cm3. The density of water is 1 g/mL. (If an object floats in water its density is less than 1 g/mL. What do you think the density will be if an object sinks in water?) Complete practice problems 1-3 on p.29 in your notes to use density to find mass.

Time Is the interval between two events The SI unit for time is the second. You may need to convert a time to seconds when solving problems. Always read the question carefully!

Temperature Is a measure of the average kinetic energy of all the particles in an object For scientific work we will use the Celsius or the Kelvin scale. To convert from Celsius to Kelvin add 273 to the Celsius reading. To convert from Kelvin to Celsius, subtract 273 from the Kelvin reading.

Assignment: Complete on loose-leaf paper. Textbook p.30 Write and answer questions 4-11. Turn this assignment in to the bin when you are finished.

Formative Assessment Activity Floating Logs Probe Activity

How reliable are measurements? Accuracy – refers to how close a measurement is to an accepted value Precision – refers to how close a series of measurements are to one another. See (and copy) figure 2-9 on p.36 Percent Error – the ratio of an error to an accepted value Percent Error = error / accepted value x 100%

Scientific Notation Expresses numbers as a multiple of two factors: A number between 1-10 Ten raised to the power (or exponent) How to: Move the decimal point to produce a factor between 1-10. Count the number of places the decimal point moved and in what direction. Remove extra zeros at the beginning or end of the number. Multiply the result by 10n where n equals the number of places moved. When the decimal moves left, n is positive. When the decimal moves right, n is negative. Complete practice problems 12-13 on p.32 in your notes.

Significant Figures All known digits plus one estimated digit. Used to indicate the precision of measurements Rules: Non-zero numbers are always significant. Zeros between non-zero numbers are always significant. All final zeros to the right of the decimal place are significant. Zeros that act as placeholders are not significant. (Convert quantities to scientific notation to remove the placeholder zeros.) Counting numbers and defined constants have an infinite number of significant figures. Complete practice problems 31-31 on p.39 in your notes.

Practice! Measurement worksheets: Metrics and Measurement Unit Conversions & Factor Label Method (dimensional analysis) Using Correct Units Scientific Notation Density

SI Olympics Using SI measurements compete with your classmates to win the SI Olympics

Graphing See Graphing Handout

Review - Read the summary on p.49. Make sure you know the key equations and relationships on p.49. Look over the vocabulary words. If a term was not covered in your notes add its definition to your notes.

Review for Test 2 Test 2 will cover measurement, scientific notation, accuracy, precision, significant figures, and graphs. A portion of this test will take place in the lab using equipment to check your ability to make precise and accurate measurements.