The Tools of Quantitative Chemistry

1. The Tools of Quantitative Chemistry Let’s Review

2. Units of Measurement • Metric System (International System of Units – SI) • All units are derived from base units; quantities expressed using prefixes

3. SI units

4. Temperature Scales • Celsius • zero is the freezing point of pure water • 100 is the boiling point of pure water • Kelvin • same size unit as Celsius • zero is the lowest temp that can be achieved (absolute zero) • no degree symbol is used with kelvin

5. Length, Volume, and Mass • SI unit for length is the meter, but usually in chemistry it is necessary to use smaller divisions, such as micrometer or nanometer. • SI unit for volume is m3, but liters are commonly used in chemistry. 1 mL = 1 cm3 1 L = 1000 mL = 1000 cm3

6. Length, Volume, and Mass • SI unit for mass it the kilogram, but smaller masses in chemistry are often used such as grams or milligrams.

7. Making Measurements • Precision indicates how well several determinations of the same quantity agree. • often expressed as standard deviation • Accuracy is the agreement of a measurement with the accepted value of the quantity. • often expressed as percent error

8. Making Measurements • Experimental Error Error = experimental value – accepted value % Error = (Error/accepted value) x 100% • Standard Deviation = the square root of the sum of the squares of the deviations for each measurement from the average divided by one less than the number of measurements.

9. Practice Problem • Two students measured the freezing point of water. Student A used an ordinary thermometer calibrated in 0.1oC units. Student B used a thermometer certified by NIST and calibrated in 0.01oC. • A: -0.3oC; 0.2oC; 0.0oC; -0.3oC • B: -0.02oC; 0.02oC, 0.00oC; 0.04oC • Calculate the average value, the percent error, and standard deviation for each student. Which is more precise? Which has a smaller error?

10. Exponential or Scientific Notation • Scientific (or exponential) notation is a way of presenting very large or very small numbers in a compact and consistent form that simplifies calculations. N x 10n exponent is positive if the number is greater than 1 and negative if the number is less than 1

11. Significant Figures • A result calculated from experimental data can be no more precise than the least precise piece of information that went into the calculation. • Significant figures are the digits in a measured quantity that were observed with the measuring device.

12. Determining Significant Figures • Rules on page 36 • Arrow Method • Addition/Subtraction • The number of decimal places in the answer is equal to the number of decimal places in the number with the fewest digits after the decimal. • Multiplication/Division • Number of sig figs in answer is equal to the quantity with the fewest sig figs.

13. Dimensional Analysis • Uses the dimensions of each unit to guide you through calculations. Conversion factors are used to change measured quantities to chemically useful information.

14. Graphing • Goal is to obtain an equation that may help us obtain new results. y = mx + b • “y” is the dependent variable; “x” is the independent variable; “m” is the slope of the line (Δy/Δx); “b” is y-intercept

15. Practice Problem • To find the mass of 50 jelly beans, we weighed several samples of beans. (see Exercise 8 on page 41) Plot these data with the number of beans as independent variable and mass as dependent. What is the slope of the line? Use your equation of a straight line to calculate the mass of exactly 50 jelly beans.

16. Homework • After reading the “Let’s Review” section, you should be able to do the following problems… • pp. 43-44 (3-6, 15-18, 21-24)