Chapter 1 Chemical Foundations Nottoway High School Dual Enrollment Chemistry 2014-2015 Dr. Gur
1.1 Chemistry: An Overview Science: A process for Understanding Nature and Its Changes.
We can explain both of these things in convincing ways using the models of chemistry and the related physical and life sciences.
What is Chemistry? Chemistry can be defined as the science that deals with the materials of the universe and the changes that these materials undergo.
1.2 The Scientific Method • Making observations. • Observations may be qualitative or quantitative.
A Qualitative observation does not involve a number.. A Quantitative observation (a measurement) involves both a number and a unit.
2. Formulating hypotheses. A Hypothesis is a possible explanation for an observation.
3. Performing experiments. An experiment is carried out to test a hypothesis.
Variables: the factors that influence the outcome of an experiment. Note: change only one variable at a time.
Independent Variable: the variable that is intentionally changed or manipulated by the experiment. (x-axis). Dependent Variable: the variable being measured or watched, sometimes called the outcome. (y-axis)
Constants all other factors which remain the same throughout an experiment. Control usually distilled water.
Repeated trials: repeat experiments until the results are consistent, usually two or three times. Conclusion: write a conclusion based on the data gathered from the experiment. The data will either support or contradict the hypothesis.
1.3 Units of Measurement A quantitative observation, or measurement, always consists of two parts: a number and a unit.
The International System of units, or the SI system, is based on the metric system.
SI units of mass, length, time, and temperature are the kilogram, meter, second, and Kelvin.
Volume (V) is the amount of three- dimensional space occupied by a substance. 1 dm3 ≡ 1 liter (L) 1 liter = 1000 cm3 = 1000 mL
1.4 Uncertainty in Measurement All measurements have a degree of uncertainty. When one is making a measurement, the custom is to record all of the certain numbers plus the first uncertain number.
Sample Exercise 1.1 Uncertainty in Measurement In analyzing a sample of polluted water, a chemist measured out a 25.00-mL water sample with a pipet (see Fig. 1.7). At another point in the analysis, the chemist used a graduated cylinder (see Fig. 1.7) to measure 25 mL of a solution. What is the difference between the measurements 25.00 mL and 25 mL?
Solution 25 mL = 24 mL – 26 mL 25.00 mL = 24.99 mL – 25.01 mL
Precision and Accuracy Accuracy refers to the agreement of a particular value with the true value. Precision refers to the degree of agreement among several measurements of the same quantity. It reflects the reproducibility of a given type of measurement.
Random error means that a measurement has an equal probability of being high or low. Systematic error occurs in the same direction each time.
Figure 1.10 • Neither accurate nor precise (large random errors). • Precise but not accurate (small random errors, large systematic error). • Bull’s eye! Both precise and accurate (small random errors, no systematic error).
Sample Exercise 1.2 Precision and Accuracy Trial Volume Shown Volume Shown by by the Graduated Buret Cylinder 1 25 mL 26.54 mL 2 25 mL 26.51 mL 3 25 mL 26.60 mL 4 25 mL 26.57 mL Average25 mL26.54 mL
Solution If the volume is 26.50 mL, the Graduated Cylinder is precise but not accurate; however, the Buret is both precise and accurate.
1.5 Significant Figures and Calculations Rules For Counting Significant Figures 1.Nonzero integers. Nonzero integers always count as significant figures. 1457 4 significant figures 4.723 significant figures
2.Zeros. a. Leading zeros are zeros that precede all the nonzero digits. They do not count as significant figures. 0.00252 significant figures 0.0003413 significant figures
b. Captive zeros are zeros between nonzero digits. They always count as significant figures. 1.0084 significant figures 0.0839085 significant figures
c. Trailing zeros are zeros at the right end of the number. They are significant only if the number contains a decimal point. 1001 significant figures 100.3 significant figures
3.Exact numbers have an unlimited number of significant figures. • Numbers that are determined by counting • 10 students • unlimited number of significant figures
b. By Definition • 1 inch ≡ 2.54 cm • unlimited number of significant figures
Sample Exercise 1.3 Significant Figures Give the number of significant figures for each of the following results. a. A student’s extraction procedure on tea yields 0.0105 g of caffeine. b. A chemist records a mass of 0.050080 g in an analysis. c. In an experiment a span of time is determined to be 8.050 x 10-3s.
Rules for Significant Figures in Mathematical Operations 1. For multiplication or division, the number of significant figures in the result is the same as the number of significant figures in the least precise measurement used in the calculation.
Corrected • 4.56 x 1.4 = 6.3846.4 • 2 4 2 • Significant Significant Significant Significant • Figures Figures Figures Figures
Corrected • 8.315/298 = 0.02790270.0279 • 4 3 6 3 • Significant Significant Significant Significant • Figures Figures Figures Figures
2. For addition or subtraction, the result has the same number of decimal places asthe least precise measurement used in the calculation.
12.11 2 decimal places • 18.0 1 decimal place • 1.013 3 decimal places • 31.123 31.11 decimal place
0.6875 4 decimal places • -) 0.1 1 decimal place • 0.5875 0.61 decimal place
Rules for Rounding 1. In a series of calculations, carry the extra digits through to the final result, then round.
2. If the digit to be removed • is less than 5, the preceding digit stays the same. • 1.331.3 • b. is equal to or greater than 5, the preceding digit is increased by 1. • 1.361.4
Sample Exercise 1.4 Significant Figures in Mathematical Operations Carry out the following mathematical operations, and give each result with the correct number of significant figures. a. (1.05 x 10-3)/6.135 b. 21 – 13.8
c. As part of a lab assignment to determine the value of the gas constant (R), a student measured the pressure (P), volume (V), and temperature (T) for a sample of gas, where R = PV T
The following values were obtained: P = 2.560 T = 275.15 V = 8.8 Calculate R to the correct number of significant figures.
1.6Dimensional Analysis We can convert from one system of units to another by a method called the unit factor method, or more commonly dimensional analysis. Unit 1 x Unit factor = Unit 2
Consider a pin measuring 2.85 centimeters in length. What is its length in inches?
The equivalence statement is • 2.54 cm ≡ 1.00 in • The unit factors are • 2.54 cm or 1.00 inch • 1.00 inch 2.54 cm
2.85 cm x 1.00 inch • 2.54 cm • = 2.85 in • 2.54 • = 1.12 in
Sample Exercise 1.5 Unit Conversions I A pencil is 7.00 in long. What is its length in centimeters?
7.00 in x 2.54 cm 1 in = (7.00)(2.54) cm = 17.78 cm = 17.8 cm
1.7Temperature Temperature can be measured on three different scales: Fahrenheit, Celsius, and Kelvin. Boiling point of water: 2120 F, 1000 C, 373 K Freezing point of water: 320 F, 00 C, 273 K