1. Bell Work
Collect your LAB-4 work sheet from graded work desk
and the Bell-Work sheet

2. Chapter-2: Analyzing Data (measurements) Dr. Chirie Sumanasekera
CHEMISTRY Notes_6
Dr. Chirie Sumanasekera
9/5/2018
TOPICS covered today:
Dimensional Analysis (contd.)
Uncertainty of data and why Sig-figs are important
Error and Percent error calculations

3. Dimensional Analysis
Dimensional Analysis (Unit Factor Method) is a problem-solving method that uses concept that any number (or factor) can be multiplied by 1 without changing its value.
Unit factors may be made from any two terms that describe the same or equivalent "amounts" of what we are interested in.
Ex: 1 inch = 2.54 cm
There are exactly  cm in 1 inch. We can make two unit factors from this information:

4. Solving Problems with Dimensional Analysis
Write down what you need to find with a question mark. Then set it equal to the information that you are given. The problem is solved by multiplying the given data and its units by the appropriate unit factors so that only the desired units are present at the end after the other units cancel out.
(1) How many centimeters are in 6.00 inches?
(2) Express 24.0 cm in inches.

5. Solving Problems with Dimensional Analysis
You can also string many unit factors together.
(3) How many seconds are in 2.0 years?
 (4) Convert 50.0 mL to liters. (This is a very common conversion.)

6. Solving Problems with Dimensional Analysis
(5) How many atoms of hydrogen can be found in 45 g of ammonia, NH3? We will need three unit factors to do this calculation, derived from the following information:
1 mole of NH3 has a mass of 17 grams.
1 mole of NH3 contains 6.02 x 1023 molecules of NH3.
1 molecule of NH3 has 3 atoms of hydrogen in it.

7. Measurements
Chapter-2:
( 34 cm)
Units
Number
Accuracy and Precision in data collection
Scientific notation
Graphical data
Dimensional analysis
Significant figures (Uncertainty in measurement)
Error & Percent Error
Base unit
Prefixes
Conversions between prefixes
Dimensional analysis

8. What is the length of this pencil in cm?
UNCERTAINTY of MEASUREMENT 21 22 24 26 23 27
29 cm 28
What is the length of this pencil in cm?
26.5 cm for sure + a bit more?
26.57 cm
The part of the measurement we guessed is shown at the end (4)
Significant figures are used to communicate the level of uncertainty in measurements

9. The last digit of a sig-fig shows the uncertainty of your data!
Expressing UNCERTAINTY of MEASUREMENT with sig-figs
(a) 30.0
(b) 30.00
(c)
(a) 30.0
(b) 30.00
(c)
= Certain up to 30.
= Certain up to 30.0
= Certain up to
The last digit of a sig-fig shows the uncertainty of your data!

10. Precision shown by significant figures Which of these is more precise?
ANSWER: (c) Is most precise because there are more figures after (right of) decimal

11. Actual Length of the pencil is 26.56 cm Measured value is 26.54 cm
Error Calculations 21 22 24 26 23 27
29 cm 28
Actual Length of the pencil is cm
Measured value is cm
Find error:
(Absolute or positive value)
ERROR = |Measured value – Actual value|
Error: |26.56 – 26.54| = 0.02 cm

12. Percent Error Calculations21 22 24 26 23 27
29 cm 28
Actual Length of the pencil is cm
Measured value is cm
Find % error:
Percent ERROR = |Measured value – Actual value| x 100
Actual value
% Error = |26.56 – 26.54| x 100 = x 100 = %

13. Percent Error Calculations
Smaller the percent error, the greater the accuracy
Example:
Comment which is more accurate: % vs %
% is smaller so it is more accurate

14. Today we examined how to perform and evaluate calculation using:
Dimensional Analysis
Significant figures
3. Error & Percent Error of data

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