**Unit 3: Measuring & Calculating**

**Unit 3: Measuring & Calculating**

BJU: Ch. 3 "Measuring and Calculating"

"Elements, Compounds, Mixtures" lab using iron and sulfur. The handout and instructions are posted near the bottom.

__Reading (Weeks 4 & 5)__BJU: Ch. 3 "Measuring and Calculating"

__Topics__- Metric system
- Conversion factors
- Significant digits
- Calculations involving the above
- The concept of 'density'

__Lab__"Elements, Compounds, Mixtures" lab using iron and sulfur. The handout and instructions are posted near the bottom.

Below: A rather dramatic demonstration of the energy which is contained in hydrogen gas (H2). Matter can exist as a gas, liquid, or solid... and a fourth category called 'plasma'. We will make small quantities of hydrogen gas - and safely study its properties - in a later lab.

__Lecture outline__

By around 1960, most nations had officially adopted the

**metric system**or

**System International (SI)**as their official measurement system. The metric system is based on "tens", and uses the meter, gram, liter, and second as basic units.

In the United States, we still use the

**Imperial System**(inches, feet, pounds, gallons) in many fields, but in Chemistry

*we use the metric system because it's easier and because it is the recognized standard*. Since Americans need to know

*both systems*, we use

**conversion factors**to convert back and forth between them. Historically, the Imperial System was used throughout the British Empire up through the mid-1900's, and since the British Empire included most of North America, we still have it.

Metric

**base units**and

**prefixes**shown below:

In the metric system, we also have

**derived units,**such as*area, volume, density, and pressure*.Density

- The density of something is its
__mass__divided by its__volume__. Density = mass/volume. - The density of water is 1,000 g/L (one thousand grams per liter).
- The density of rock is around 2,000 g/L. Therefore, a rock will sink in a beaker of water.

Accuracy vs. Precision

Significant Figures

- Accuracy is how close you are to the "true" value. Shooting an arrow directly in the bullseye is being 'accurate'.
- Precision has to do with 'repeatability', or how tightly grouped your measurements are. Shooting 10 arrows near the bullseye is 'precision'.
- Precision is also an indication of how small the scale 'gradations' are on your measuring instrument. A measurement like 5.125 L is "more precise" than 5.1 L, for example (even though neither one is perhaps very "accurate").

Significant Figures

- We have to be careful how many digits we show in our answer.
- For addition & subtraction, you can keep as many place values in your answer as the least precise measurement contains
- For multiplication & division, you can keep as many significant figures in your answer as the measurement with the least significant figures has

Scientific Notation

- Chemistry uses very small - and also very large - numbers. We use scientific notation to write these numbers.
- For example, 6 x 10^3 means a 6 with 3 zeroes, or 6,000.... 6 x 10^23 means a 6 with 23 zeroes!... which is a really huge number.

__Elements, compounds, and mixtures lab__

- We will start off by observing and testing the properties of two common elements, iron (Fe) and sulfur (S). This includes dissolving the iron in acid and trying to dissolve (unsuccessfully) the pure sulfur in various things.
- Then we will carry out a reaction which combines the Fe and S to form FeS, a completely new compound with a different appearance and properties.
- After that, we will react a small amount of the FeS with hydrochloric acid (HCl) to form hydrogen sulfide gas (H2S) which has quite different (and noticeable!) properties.
- Finally, we will bubble some of the H2S into a water sample, to demonstrate one way you could remove heavy metals from wastewater.
- All of this serves as an excellent introduction to elements and compounds, and setting up chemical reactions.

2._elements_compounds_mixtures_lab_handout_.pdf |

__Homework__

Your class emails will give the due dates and instructions for homework. Don't work ahead, and don't do homework problems which haven't been assigned! As a general rule, you will have one homework assignment due each week.

2._density_problems__air_in_classroom_and_steel_ball_.docx |

2._measurement_homework__problems1.docx |

2._measurement_homework_problems_student_exemplar.pdf |