In measurement, numbers by themselves often lack real meaning. For example, 4 grams (g) and 4 liters (L) both have the number 4, but they are not the same quantity. The "unit" or "label" provides a context for the number. We can compare 4 grams and 5 grams. We can't compare 4 grams and 5 liters.
Sometimes, there are relationships between different units. The density of water is 1,000 grams per liter. That means, for water, 1,000g = 1 L. That type of equality between different units is called a definition. Definitions such as these can be used to do unit conversions. In this lesson, you will learn to:
- Create conversion factors from definitions
- Select the correct conversion factor to make units factor or cancel, and
- Do conversions by factor label
You will also become familiar with metric units and prefixes. Factor label will enable you to convert a metric unit with one prefix to an equivalent unit with a different prefix.
Lesson summary and worksheet review.
Scientific notation is a convenient way to express very large or very small numbers. This lesson will explain how to convert from standard notation to scientific notation and from scientific notation to standard notation. It will also explain how to do calculations using scientific notation. You will learn:
- The proper format for scientific notation
- How to move the decimal and adjust the exponent to convert notations
- How to multiply, divide, add, and subtract numbers with exponents
These skills will be useful for other quantitative work in chemistry.
Lesson summary and worksheet review
Observation is a key process in science, but our senses can be misleading. Scientist use measurement to improve observation, but measurements have limitations too. Observations must be evaluated for accuracy and precision. This lesson will teach you:
- The definition of accuracy
- The definition of precision
- How to evaluate a measurement for accuracy and precision
For now, this is a qualitative concept. In later lessons, errors will be evaluated quantitatively.
Lesson summary and worksheet review
A measurement such as 35.1 m has two measured values and one estimated value. A measurement can only have one estimated value. It is always the last digit. In 35.1 mm, the 3 and the 5 are measured values, and the 1 is estimated. If 35.1 mm is converted into millimeters, 35,100 mmthe 3 and the 5 are still the measured values, and the 1 is still estimated. The zeros are place holders, and are not significant figures.
When students do calculations with measurements, estimated values are frequently multiplied, divided, added, or subtracted by and from other estimated values. How many decimal places should be in the answer so that there is still only one estimated digit? Your calculator does not know. It gives you lots of decimal places in your computations. It is your job to round the result to something sensible. Significant figures help you do just that. In this lesson, you will learn:
- How to distinguish significant figures from place holders
- How to count the number of significant figures in a measurement
- How to round the results of calculations to the correct number of significant figures
This skill will ultimately help you to arrive at sensible answers from your calculations.
Lesson summary and worksheet review
Errors of measurement are unavoidable. This is because measuring devices have their limitations. But how bad are the errors they cause? It's not just the size of the error that matters. It's the size of the error compared to the size of what is measured. In this lesson, you will learn to:
- Calculate the absolute error, and
- Calculate the percentage error
The percentage error makes the comparison between the absolute error (the size of the error) and the size of what is being measured.
Lesson summary and worksheet review.
Atomic mass is measured by a mass spectrometer. The data from a mass spectrometer comes in the form of masses and percentages. This makes it possible to calculate a weighted average. In this lesson, you will learn to:
- Understand the similarity between the standard way of calculating an average and a weighted average
- Understand why average atomic masses are weighted averages, and
- Calculate average atomic mass from masses and their respective percentages
Lesson summary and worksheet review.
