Chemistry – Nature of Science

  • Scientific method—a systematic and orderly approach to solving a problem
  • 6 steps of the scientific method:

1) State the problem or pose a question

2) Form a hypothesis (an educated guess)

3) Experiment

4) Collect and interpret data

5) Draw a conclusion

6) Communicate the findings

  • The conclusion should support or reject the hypothesis.
  • Variable—part of the experiment that changes
  • Controls—parts of the experiment that stay the same
  • Independent variable—the factor that a scientist changes during an experiment; also called the manipulated variable
  • Dependent variable—the factor that changes as a result of changes to the independent variable; also called the responding variable

Controlled experiment—experiment in which only one factor changes at a time

Chemistry – Measurement

A. Accuracy vs. Precision

Accuracy - how close a measurement is to the accepted value

Precision - how close a series of measurements are to each other

B. Percent Error - Indicates accuracy of a measurement

C. Significant Figures

- Indicate precision of a measurement.

- Sig figs in a measurement include the known digits plus a final estimated digit

- Counting Sig Figs

Count all numbers EXCEPT:

Leading zeros -- 0.0025

Trailing zeros without
a decimal point -- 2,500

-Calculating with Sig Figs

Multiply/Divide - The # with the fewest sig figs determines the # of sig figs in the answer.

Add/Subtract - The # with the lowest decimal value determines the place of the last sig fig in the answer.

Exact Numbers do not limit the # of sig figs in the answer.

Counting numbers: 12 students

Exact conversions: 1 m = 100 cm

“1” in any conversion: 1 in = 2.54 cm

D. Scientific Notation

- Converting into Sci. Notation:

Move decimal until there’s 1 digit to its left.

Places moved = exponent.

Large # (>1)  positive exponent
Small # (<1)  negative exponent

Only include sig figs.

E. Proportions

Directly Proportional - increasing or decreasing together

Inversely Proportional - means that one variable goes up while the other goes down

Chemistry – Measurement (Value + Unit)

C. Derived Units - Combination of base units.

Ex: Volume (m3 or cm3)

lengthlengthlength

Calculating Density:

An object has a volume of 825 cm3 and a density of 13.6 g/cm3.

Find its mass.

A liquid has a density of 0.87 g/mL. What volume is occupied by

25 g of the liquid?

Chemistry - Unit Conversions

A. SI Prefix Conversions

- Find the difference between the exponents of the two prefixes.

- Move the decimal that many places.

- Going from a smaller unit to a larger, move the decimal to the left.

- Going from a larger unit to a smaller, move the decimal to the right.

B. Dimensional (Unit) Analysis

Units, or “labels” are canceled, or “factored” out

Steps:

1. Identify starting & ending units.

2. Line up conversion factors so units cancel.

3. Multiply all top numbers & divide by each bottom number.

4. Check units & answer.

1. How many milliliters are in 1.00 quart of milk?

2. You have 1.5 pounds of gold. Find its volume

in cm3 if the density of gold is 19.3 g/cm3.

3. How many liters of water would fill a container

that measures 75.0 in3?

4. Your European hairdresser wants to cut your

hair 8.0 cm shorter. How many inches will he be

cutting off?

5. Taft football needs 550 cm for a 1st down.

How many yards is this?