Chemistry 11 Unit 1: SI units and unit conversion (Hebden p. 9-40) SI units "Systeme International", or SI Units, is a standardized system of measurement based on internationally agreed definitions. There are several base units. Quantity Unit Symbol Length Mass Amount of substance mole mol Time Temperature Energy joule J Electric current ampere A Volume are made by using multiple the base units. For example: Velocity = with a unit (m / s) Density = with a unit (kg / L) Molarity (Concentration) = with a unit (mol / L) What do you notice about units like ml (mili litre), µm (micro meter), ηs (nano second)? These units are made of and base unit. Using a prefix and base unit tougher allow us to work very or numbers more conveniently. Each prefix represents a multiple of 10 x.
(Memorize:,,,,, and for the rest of the year!) Explore the scale of universe (http://htwins.net/scale2/) Ms. Yajima is about 157cm in height. How many m is that? 157 cm = 1.57 m Can you fill in the blank to show the step? 157 cm X m = 1.57 m cm E.g., Use the same method to convert an average height of minions from 1050 mm into m. 1050 mm X m = m mm E.g., How many g are there in an average mass of bananas (0.116 kg)? 0.116 kg X g = g kg The fraction used above to change one unit into another is called the unit conversion factor. Unit conversion factors represent relationship between two units. E.g., Write unit conversion factor between the following units: 1. cm and m 3. µg and g 2. mm and m 4. ηs and s Unit conversion between two non- base units requires multiple factors in the step. First, change the given unit to non- prefix unit, then change to the desired unit. E.g., How many kg are there in an average mass of an adult human brain (1300000mg)? 1300000mg X g X kg= kg mg g 2
Q: How many μg are there in 7kg? (Use two conversion factors because it is a conversion between two non- base units. Do kg à g à μg) Q: Convert 72 μs into ms. (Use two conversion factors because it is a conversion between two non- base units. Do μs à s à ms) Challenge Q: Convert 100m/μs into km/s. (Do m à km, and μs à s ) Non- metric Unit Conversions Q: How many minutes are there in 2 hours? What is the conversion factor here? ( i.e., What is the relationship between hours and minutes?) à Begin with the value that is given. 2 hours X = Note: You may not always know the conversion factor. In that case, read the question carefully to unlock the secret message. You can still follow the same steps. Q: If a car can go 80 km in 1 hr, how far can the car go in 8.5 hr? Q: If 0.200 ml of gold has a mass of 3.86 g, what is the mass of 5.00 ml of gold? 3
Density Density is an example of derived quantity. It is a number made by combining two or more other values. Density is defined as mass divided by volume (or mass over volume). In other words: density = In the lab experiment, you will find the density of a given sample then compare your result with the accepted value. Reading a Value Step 1 When reading a value off glassware (or other instruments), first figure out. For example, in this graduated cylinder, each spacing represents 1 ml. Step 2 Read the value using the of the It is approximately 36mL, but it is a little bit more. How much more? NOTE: Generally, depending on how visibly apart the spacings are, you can estimate the last digit to 1/10 th to 1/5 th (or even less) of the spacings The bottom of the meniscus looks like it as exactly half- way between 36 ml and 37 ml. Therefore, the reading is ml Q. Read the burette reading. Each spacing is ml. But the number increases going down. Wow! Estimated value is ml, but it is a bit more than ml. The bottom of the meniscus looks like slightly below 1.4mL, so I am going to read it as 1.41mL. But you may read it as 1.42mL, which is acceptable. Answer such as 1mL, 1.4mL, 1.410mL, 1.415mL are NOT acceptable. Acceptable values are ml, ml or ml. Notice the third number (3 rd significant figure) is not really fixed so it is called the digit. In this reading, we have 2 certain digits, and 1 uncertain digit NOTE: In most of your readings, every digit is certain except for the last one. 4
Significant Figures At an Olympic trials race, a runner was claimed to have crossed the finish line at a time of 9.6916847 seconds. A stopwatch was used to time the runner during the race. What is wrong with the recorded runner s time? If the stopwatch can only read to 0.1 s, then it is silly to claim that the time is 9.6916847 s. The stopwatch can t measure the time to 7 decimal places. Therefore, the last digits (916847) have no significance! They are not important. What is a significant figure? Significant figure: is a measured and digit. Rules of Counting Significant f=figures: 1. Any non zero number is a significant figure. Eg. 35.2 This value has sig figs. 2. Leading zeroes are NOT significant because it is there to place the decimal point. Eg. 0.025 ( sig figs) 3. Zeroes in between non zero numbers are significant. Eg. 205 ( sig figs) 20005 ( sig figs) 4. Trailing zeroes (the ending zeros ) are significant..? Eg. 25.00 ( sig figs) 25.0000 ( sig figs) Hmmm well. Trailing zeroes are ONLY significant when a decimal point is shown. (we assume that the last digits are zeroes because they are rounded off) Eg. 0.1 ( sig figs) 0.10 ( sig figs) 10 ( sig figs) 10. ( sig figs) 1100 ( sig figs) 1100. ( sig figs) 12500 ( sig figs) 5. Defined values have number of sig figs. Eg. 1 km = 10 3 m (infinite number of sig figs) Eg. Dozen = 12 (infinite number of sig figs) Scientific Notation: From decimal point of the original number, count how many times the point needs to move to get only one digit left of the point. Move left = exponent Move right = exponent 5
Eg. Change 358123 into scientific notation. Eg. Change 0.1135 into scientific notation. Note: Only count the number of significant figures in the part of the scientific notation. Rounding the Numbers to the Correct Number of Sig Figs Start cutting digits from the right and decide which last digit to keep. Leave it as is if the next digit on the right is less than 5. Increase it by 1 (rounding up) if the next digit on the right is more than 5. 28.62 2.8354 Q. Round to 3 significant figures: 28.999 à 123456 19999 Multiplying and Dividing Numbers: After multiplying or dividing numbers, round off the answer to the LEAST number of significant figures contained in the calculation. Eg. 2.00 X 3.000 = 6.00 3 s.f. 4 s.f. 3 s.f. NOTE: You should keep all your digits used on your calculator during the calculations. Only the final answer should be rounded!!! Adding and Subtracting Numbers: After adding or subtracting numbers, round off the answer to the LEAST number of decimal places contained in the calculation. Eg. Add 12.56 and 125.8 together. Eg. 12.5 6 2 decimal places + 125.8 1 decimal place 138.3 6 138.4 final answer rounded to 1 decimal place! 6
Accuracy and Precision In English class, both these terms mean very similar things; but in science it is not so! is determined by how close your value is to the TRUE or CORRECT value. is determined by how many s.f. your value has and by how often your values are repeated. Eg. When measuring the density of silver sample, density was measured to be 8.1g/mL, 8.0g/mL and 7.8g/mL. If the correct density of silver is 10.5 g/ml, are your values precise, accurate or neither? Try the following example to determine if it is accurate, precise, both or neither. 7