Choose the right equipment for lab work. Following Rules for Precision and Accuracy. Following Significant Figure Rules

Transcription

1 Chemistry is a Quantitative Science Part I Measuring Matter At the end of Part I you should be able to: Choose the right equipment for lab work Make accurate measurements Following Rules for Precision and Accuracy Record data with units Following Significant Figure Rules Use data in calculations Showing units Using the Units Conversion Meth Following Significant Figure Rules

2 Measuring Matter and Following Best Practices for Precision and Accuracy Chemistry is the branch of science that deals with the study of the composition, structure and properties of matter and the changes it undergoes. What is Matter? Matter is anything that has mass and takes up space. In Biology and Physics you studied specific aspects and interactions of matter. In the past what are the different ways you have observed and/or classified matter? (Not sure think about a specific lab) What variables did you observe and or measure in that study of matter? What units are used for these variables?

3 What are Precision and Accuracy? Accuracy Accuracy is how close a measured value is to the actual value. The actual value is also called the true or accepted value. It can be looked up. Think: closeness Precision Precision refers to the closeness to each other, of two or more measurements of the same subject using the same device. Think: Repeatable In summary, accuracy describes the difference between the measurement and the part's actual value, while precision describes the variation you see when you measure the same part repeatedly with the same device. It is possible for a measurement to be accurate, but not precise or for a measurement to be precise, but not accurate. The goal is to be both accurate and precise. Consider these examples: A) Goal is to hit the center dot of the target

4 B) An object has a confirmed mass of 32 grams In lab students collected the following data for the mass of this object: 40 g 55 g 32g and 50 grams How would you rate their precision and accuracy? C) A sample has an accepted value for density of 0.31 g/cm 3 In lab students collected the following data for this sample: 0.30 g/cm g/cm g/cm g/cm 3 How would you rate their precision and accuracy?

5 Choosing the Right Equipment Use your past experience, following instructions & ask questions. The task as well as precision and accuracy determine the choices

6 Using Electronic Balances *Steps to follow Be sure pan is clean and dry Turn on and wait for it to warm up Should see all zeroes Use a weighing boat *Use a weighing boat How to record data Why are ALL numbers including trailing zeros recorded?

7 What are significant figures and why do we use them? It is important to be honest when reporting a measurement. In your report, you cannot make the measurement appear more accurate or precise the equipment used to make the measurement allows. We can achieve this by controlling the number of digits, or significant figures, used to report the measurement.

8 Consider these examples: A) B)

9 Significant Figures Exact vs Measured Numbers Exact numbers are those that have defined values or are integers that result from counting numbers of objects. For example, by definition, there are exactly 12 eggs in a dozen, exactly g in a kilogram, and exactly 2.54 cm in an inch. Do you recognize these values as conversion factors?

10 Identifying the Number of Significant Figures in a Given Value Some people say there are three rules on determining how many significant figures are in a number: *Non-zero digits are always significant. *Any zeros between two significant digits are significant. *A final zero or trailing zeros in the decimal portion ONLY are significant.

11 Calculations and Significant Figures Two sets of rules 1) Addition and Subtraction 2) Multiplication and Division

12 Helpful Videos for Significant Figures These videos were listed on your summer assignment. If you find any other helpful videos please let me know. *Why Are Sig Figs Important *How to Determine Sig Figs *Sig figs and Zeroes *Sig figs with Addition and Subtraction *Sig figs with Multiplication and Division *Sig figs and Scientific Notation