QUESTION OF THE DAY: 1. Write the number 295 to two significant figures. 2. How many significant figures does the number 610. have? 3. Use the ruler below to determine the length of the strip of metal using correct number of significant figures.

Remember When a measurement is recorded properly, all of the digits that are read directly (certain) and one estimated (uncertain) digit are called significant digits. The number of allowable significant digits is determined by the marks or gradations of the instrument. Sometimes a “0” is the estimated digit and must be recorded.

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Your Turn

Quiz on Friday Scientific Notation Significant Figures Metric Conversions Accuracy and Precision

LAB SAFETY

Video http://www.youtube.com/watch?v=xJG0ir9nDtc&list=PL50300782DFB0AECF After watching the video, what are 10 safety procedures you learned? Be prepared to discuss.

Video http://www.youtube.com/watch?v=6aK2CKrdjbE After watching the video, what are 10 safety issues that you saw? Be prepared to discuss them.

Books Closed - QUIZ 1. What should you be wearing on your face at all times during the lab? 2. Where is the eye wash station located in the room? 3. What are two things you should always do before leaving the lab? 4. What should you do if glassware breaks or glassware is cracked? 5. What is never allowed in the lab?

NOTE OF THE DAY: THIS WEEK IN SCIENCE ASSIGNMENT Every week two people will be selected to present a current news topic in the world of science. Research something that interests you and is appropriate for class. Give approximately a two minute presentation on your findings and be prepared to answer questions. Students will present every Tuesday until Christmas. Marking Guide /5 Student is knowledgeable of topic. DON’T just read your summary! /1 Student includes what it is, why they find it interesting, and where they got their information. /3 Student hands-in a minimum of 1 page (double – spaced) write up. This includes the what, the why and the source. /1

TOMORROW When you come into class the lab instructions will be on your desk. It is expected of you to read over the instructions and become familiar with the procedure. This is in place of the QUESTION OF THE DAY. FRIDAY On Friday there will be a quiz on scientific notation, metric conversions, significant figures, and accuracy and precision. Yesterday If you weren’t here yesterday then you missed a quiz. It is up to you to come see me after class in order to set up a time to write the quiz. Powerpoints are on the wiki. chemistry202013.wikispaces.com

Precision, Accuracy, and Type of Error Precision, Accuracy & Types of Error Precision refers to how close a series of measurements are to one another; precise measurements show little variation over a series of trials but may not be accurate. Accuracy refers to how close a measured value is to an accepted value. Imagine you are doing an experiment with 10 individual readings. To have high precision those 10 readings all have to be close to each other. To have high accuracy those readings must be close to the "true value" or the correct answer.

Error There are two types of error that are commonly encountered in the lab random and systemic error. Random error occurs for many reasons and is random. Each time you take a measurement you have to decide what that measurement is and each time it is and each time it can change slightly. To compensate for random error multiple trials are completed. While you may still take measurements differently each time the average will be a balance of high or low readings. Using averages is the only way to get low precision but high accuracy measurements.

The second type of error in the lab is systematic error The second type of error in the lab is systematic error. This error arises in uncalibrated equipment or unfamiliarity with that equipment. If you were to use a scale that you did not calibrate and was off by 1.000 gram then all your results would be off too. There would also be error if you were to take readings in a burette wrong (from the top of the meniscus instead of the bottom).

Average Average, also referred to as the mean, is calculated by taking the sum of a set of numbers and dividing it by how many numbers you have. For example, if I want to calculate the mean of 12, 13, 14, and 20 I would: Step 1: Find the sum of the numbers. 12 + 13 + 14 + 20 = 59 Step 2: Divide the sum by the amount of numbers used. 59 ÷ 4 = 14.75 14.75 is the average or the mean.

Diameter

Volume Common measurement of volume in chemistry is in mL or L. When measuring volume it is important to know what the units the instrument you are using uses. When measuring volume you DO NOT measure from the highest point of the liquid. Liquid usually forms a half moon which can make it difficult to read. This half moon is called the meniscus. When taking a measurement it is important to measure form the bottom of the meniscus, or at the lowest point of the half moon.

In your notes draw a picture of this in the box provided.

Density Density is a widely used characteristic used to characterize a substance. It is the amount of mass per unit of volume, for example, 1.00 g/mL. Or, density can be thought of as the compactness of the substance. Keep in mind that density changes based on the temperature (think of ice) so the temperature should be reported as well. For example ethanol's density at 25 Celsius is 0.79 g/mL.

Calculating Density Density is calculated by dividing the mass of a substance by the volume of the substance, this is where g/mL is derived. For example, if the mass of a substance is 13.4 g and the volume of the substance is 20. ml, then what is the density?

Calculate the density of a substance whose mass is 110 Calculate the density of a substance whose mass is 110. g and volume is 119 ml.

Calculating Percent Difference Percent difference helps us to determine how accurate our measurements are. Percent Difference measures accuracy. Or, think of as “How much you’re off by, divided by actual.”

Example: Jim is working in the lab to determine the density of water at 25 °C. He makes three measurements which are 0.8972 g/mL , 0.8974 g/mL & 0.8973 g/mL. He wants to know how close he is and find out that the true density is 0.9970 g/mL. Is Jim's work accurate or precise?

What is the percent difference if you measure the density of an object to be 32g/cm3 and the actual density of the object is 36 g/cm3.

Lab Report Purpose: What are you trying to determine, prove, observe, etc? Materials: What materials or equipment are you using to complete your purpose? Procedure: What are the steps are you completing to achieve your purpose? In many cases this section will just need a reference to the lab handouts. Experimental Data: What results did you get? This section does not explain the results, just recording of what you measured or saw during the experiment. This is where you will make all your experimental observations.

Questions: Answer the questions using the experimental data. Conclusion: Did you get the answer you were looking for? What was it? If there are questions on the lab handout make sure those are answered in your conclusion or listed somewhere. This section should also include any sources of error.

Equipment Name Picture How do I use it? Beaker Beaker is used to measure volume of a liquid. Pour the desired amount of liquid to the desired line. Burette Use the stopcock to dispense the desire amount of liquid. Note: The 0.00 ml mark on a burette is at the top and not the bottom. Keep this in mind when you are measuring liquids using this instrument.

Balance Used to measure the mass of an object or substance. Make sure the scale is zeroed before massing an object. Graduated Cylinder Used to measure the volume of a liquid in ml. This instrument should only be used when it is placed on a flat surface.

Thermometer When taking the temperature of a liquid you want to ensure that the entire bottom of the thermometer is submerged in water and the thermometer is not touching the glassware. Using a thermometer, temperature is measured in 0C.