Entry task: Block #2 Sept 26 th -27 th

Entry task: Block #2 Sept 26th-27th Write down Question: Which measurement is more precise? AND WHY!! 2.3 mls or 2.31 mls You have 5 minutes!!

Agenda • Sign off Volume Lab • Discuss findings and errors • Significant Figures- NOTES!!

VOLUME ACTIVITIES-- 1. Start with the 10 ml graduated cylinder 2. What is the graduation marks on the 10 ml graduated cylinder?____________ 3. Do this for the 25ml , 50 ml and 100 ml graduated cylinders and record the graduation marks below. 25 ml__________ 50 ml______________100ml__________

4 Starting with the 10 ml graduated cylinder, place some water into the graduated cylinder and record the volume here _____________. 5. Practice reading the volumes (both you and your partner) with the other sizes of graduated cylinders. 6. Record the volumes below. Water in a 25 ml_________ Water in a 50 ml___________ Water in a 100ml________

4. Burets are the most accurate measuring tool for volume that we use in chemistry. Note that the buret is assembled for you. Before adding liquids, make sure the stopcock, the turning devise at the bottom is “off”- meaning that the knob is perpendicular to the buret. Using a funnel, add water – slowly until you have almost filled it to the top.

Notice that the markings are reversed from the graduated cylinders. Place a small beaker beneath the tip of the buret. Place thumb and forefinger around the handle to turn the stopcock. This will align a hole in the stopcock to the buret allowing liquids to flow. Slowly turn stopcock to lower the level of water to hit- zero!! What are the graduation marks for the buret?___________

Using a buret (very accurate), dispense exactly 25 ml of water into a 250 ml calibrated beaker. Read the calibrated beaker. Record what the beaker says__________________ • How accurate a measurement does the beaker give? EXPLAIN!! (1pt) ANSWERS MAY VARY- Not accurate at all. The volume was lower (higher) in the beaker.

Repeat the same procedure with a 25 ml and a 50 ml graduated cylinder. Record what the 25 ml beaker says_____ 50 ml_____ How accurate a measurement does the 25 ml and 50 ml graduated cylinder assuming great accuracy of the buret? In the 25 ml graduated cylinder, the volume of 25 mls from the buret was higher- almost 27 mls In the 50 ml graduated cylinder, the volume of 25 mls from the buret was higher- almost 30 mls

Summarize the accuracy of the beaker and the two graduates as compared to the buret: (2 pt) The larger the graduated cylinder, the less accurate the measurement. With 25 mls grad cyl. it was closer to 27mls and with the 50 ml grad cyl. it was 30 mls.

Using a plastic pipette, count the number of drops needed to fill a 10 ml graduated cylinder to the 2ml mark. Repeat this two more times and record your data on the answer sheet. ( 1/2 point for each data entry) 1st __________ 2nd____________3rd ____________ Add your trials and divide by 3 to get average. How many drops(average) ________ = 2 ml One ml = _______ drops One drop = _________ ml (do the math) Answers May Vary ~50 ~25 • 0.04 ml 1 ml 1 drop = 0.04 ml OR 4.0 x 10-2 ml 25 drop

6. Being the very careful analytical scientist that you are, number the following glassware pieces you would use in order of increasing accuracy. The most accurate piece has the highest number ( 1/2 point for each answer). A) You want to measure 22 mls of solution. _____50 ml graduated cylinder _____50 ml non graduated beaker _____25 ml graduated cylinder _____50 ml graduated beaker 3 1 4 2

6. Being the very careful analytical scientist that you are, number the following glassware pieces you would use in order of increasing accuracy. The most accurate piece has the highest number ( 1/2 point for each answer). B) You want to measure 11 mls of water _____ a 25 ml buret _____25 ml graduated beaker _____25 ml graduated cylinder _____50 ml graduated cylinder 4 2 3 1

Buret?? YES. How do you know?? The measurements are upside down Graduations are 0.1 ml Measurement 1.40 ml

Buret?? NO How do you know?? The measurements are not upside down Graduations are 0.2 ml Measurement 6.61 ml

Buret?? YES. How do you know?? The measurements are upside down Graduations are 0.1 ml Measurement 21.30 ml

Volume Measurements 1. If you were asked to measure exactly 1 ml you would use a ________________. 2. Most precise volumetric measurements are made by using a _________or a _____________. 1st choice- buret 2nd choice- 10 ml grad cylinder 3rd choice - pipet Buret pipet

Volume Measurements 4. You used 47 drops of water dispensed with a dropper. The dropper was calibrated to 15 drops = 1 ml. How many mls did you use? 1 ml 47 drop 3.13 mls 15 drop

Volume Measurements 5. What is the difference between a 250 ml calibrated and a 250 ml non calibrated beaker? In what type of situation would you use each type of beaker? A calibrated beaker can give rough estimates of various amounts of liquids up to 250 ml. A non calibrated beaker has no calibrations and is not used for volume determination, but can be used to contain various chemical reactions.

Significant Figures & SciNotation

I can… • Use percent error to describe the accuracy of experimental data • Use significant figures and rounding to reflect the certainty of data

% Error- KNOW THIS!! Percent error is the ratio of the experimental value to an theoretical value. Experimental value is the data collected or found through experimental means. Theoretical value is the data that is most accepted by scientist, proven by mathematical means or “in the book”. Narnia!!

% error Percent error is the ratio of an error to an accepted value Percent error = experimental - theoretical theoretical value X 100 In our lab, what was the theoretical value or accepted (most correct) volume? From the buret, was 25 mls of water Percent error = • 30 mls-25mls X 100 • 25 mls • 0.2 x 100 = 20%

So what does ± mean The ± (plus/minus) means that the glassware is either 5% more than the calibrated amount or less than the calibrated amount. 100 mls in this beaker could be 95 mls in a buret or 105 in a buret.

2 2 1 1 0 0 Significant Figures Significant Figures are used in laboratory measurements to denote the accuracy of the instrument being used to make the measurement. Centimeter ruler A Centimeter ruler B 1. We know that the clip is greater than 1 cm but less than 2 cm. 1. We know that the clip is greater than 1.8 cm but less than 1.9 cm. 2. We know that the clip is more than 1.5 cm, closer to 2 cm but less than 2 cm. 2. We know that the clip is more than 1.8 cm and halfway to 1.9 cm. 1.8 cm 1.85 cm We’re not certain about the .8 We’re not certain about the .05

So what does that mean? 1.8 cm v. 1.85 cm In Science, the ruler that was most accurate is B. Also there are more digits to make it more significant. Ruler A only had 2 digits (1.8 cm) Ruler B only had 3 digits (1.85 cm) The more digits in the measurement the more precise the measurement tool is.

Provide the measurements Look at the centimeter ruler. How many significant figures should your answer have? One sig fig for the centimeter place other sig dig to the millimeters place Last digit would be uncertain. 3 significant digits. 3.00 cm 1.54 cm 0.90 cm 3.50 cm

SIGNIFICANT FIGURE RULES Digits other than zero are always significant. 96g 2 sig. fig 61.3g 3 sig. fig 0.52g 2 sig. fig 1345cm 4 sig fig One or more FINAL zeros used AFTER the decimal point are always significant. 4.7200 km 5 sig. fig 3 sig. fig 82.0 ml 2 sig. fig 4 sig. fig 3.0 cg 145.0 hl

SIGNIFICANT FIGURE RULES Zeros between two other significant figures are always significant. 5.029 m 4 sig. fig 306 mg 3 sig. fig 4 sig. fig 56.701 g 5 sig. fig 0.3040 ml Zeros used ONLY as placeholders for the decimal are may or may not be significant. 7000 g 1 or 4 sig.fig 10.00 cm 4 sig.fig 4 sig.fig 0.00736 mg 3 sig.fig 0.002030 dl

Practicing Significant Figures How many significant figures? 5.784 g 4 s.d. 0.0035 cm 2 s.d. 73900 m 3 s.d. 0.9320 L 4 s.d. 1.090 hm 4 s.d. 130.0 mg 4 s.d. 5 ml 0.09030 kl 4 s.d. Unlimited s.d.

Entry task: Block #2 Sept 26 th -27 th