Ms. Shields Chem I Unit 1 Notes

Scientific Method: a way of answering questions about the world

¬1. State the Problem

¬2. Collect Observations

¬3. Search for Scientific Laws

¬4. Forming hypothesis

¬5. Test hypothesis with Experiments

¬6. Forming theories

¬7. Modifying theories

Scientific Method Continued

¬ Hypothesis

– MUST be testable

– Carefully devised procedure for making observations and collecting data

¬ Variable: Factor that is being tested

¬ Experimental Control: responds in an expected way

¬ Natural Law: Draws together the results of many observations and experiments

¬ Theory: Explains what the results mean

Significant Figures
A way to report the most accurate numbers involved in scientific data.

– Because scientists have to maintain accurate records on all experiments they perform, it is vital to report the most accurate numbers when reporting data. Because of this, the recording procedures using SIGNIFICANT FIGURES was devised.

5 Generalized Rules for Sig Figs

¬ Any non zero number is always significant and is counted

a. 4578 has 4 significant figures

¬ Zeros between non-zeroes are always significant and are counted

a. 503 has 3 significant figures

¬ Trailing zeroes with a decimal are always significant

a. 7000.0 has 5 significant figures

¬ Trailing zeroes without a decimal are never significant

a. 1000 has 1 significant figure

¬ Leading zeroes are never significant

a. 0.005 has 1 significant figure

Processes when adding and subtracting are different from other types of math functions

Specific Rules

– Identify the number in the problem which has the least number of digits PAST the decimal point.

– Your answer must have the exact same number of digits past the decimal point as the number you identified in step 1.

1.02554 + 5.034508 =

(4 sig figs) + (5 sig figs) = value with 4 sig figs after decimal

1.02554 + 5.034508 = 6.060048 (actual)

¬ RECORD: 6.06005 as the answer.

Specific Rules for Multiplying and Dividing

¬ Identify the number in the problem with the fewest number of sig figs (in the ENTIRE number, not just after the decimal)

¬ Your answer must have exactly the same number of sig figs as the number you identified in step 1

¬ 250 * 12.75 =

¬ (2 sig figs) * ( 4 sig figs) = value with 2 sig figs total

¬ 250 * 12.75 = 3187.5 (actual)

¬ RECORD: 3200 as the answer

Rounding when using Significant Figures

¬ Find the amount of significant figures that will be recorded as the answer.

¬ Go to the last number of the sig fig and the number directly after it.

¬ If the number directly following the last sig fig is 4 or LESS, the value of the sig fig stays the same. If it is GREATER round the value to the next higher number.

¬ 4536.1578 rounded to five sig figs

¬ Look only at the 1 and 5

¬ 5 is higher , so round 1 up to 2

¬ Recorded value is 4536.2

The International System of Units (SI) – Alias – Metric System

SI Base Units

Quantity

Name

Symbol

Length

meter

Mass

kilogram

Time

second

s or sec

Amount of substance

Mole

mol

Thermo-dynamic Temperature

Kelvin

Electric Current

Ampere

Luminous Intensity

Candela

Metric Prefixes

Prefix

Symbol

Multiplication Factor

Mega

x 1,000,000

Kilo

x 1,000

Deca

dc or dk

x 10

Deci

x 0.1

Centi

x 0.01

Milli

x 0.001

Micro

μ or u

x 0.000 001

Nano

x 0.000 000 001

¬Derived Units: an SI unit made from a combination of base units

Example: Find the volume of a rectangular object (l = 2m, h = 1m, w = 2m)

Volume = lwh = 2m * 1m * 2m = 4m³

m³ = a derived unit for volume

Factor Label Method

¬ Step 1 Write known value as a fraction with a one underneath

¬ Step 2 Look for a conversion from the unit you begin with to the one that you wish to get to.

¬ Step 3 Place these on top and underneath so that the unit to the known value cancels out.

¬ Step 4 Multiply straight across the top and bottom.

¬ Example: To convert from 100 mL to cL

¬ 100 mL * 1 cL = 10 cL

¬ 1 10 mL

Scientific Notation

¬ Use for very large and very

¬ Number is separated into 2 parts

– A number (1-9)

– An exponent (the power of 10)

¬ Example:

6945487 is written as 6.945484 X 10^6.

Sig Figs and Scientific Notation

• Only the significant figures are reported in a value, with the rest being placed in the exponent as a placeholder, but not considered significant.

Example:

6945487 with three significant figures is recorded as 6.94 X 106

Uncertainty in Measurement

¬ The last digit in a recorded measurement is the estimated (uncertain) digit

– Digital Display: The last digit is estimated

– Scales: Estimate and record one digit more than the smallest marking.

¬ Measurements are uncertain because:

– Instruments are never completely free of flaws

– Measuring always involves some estimation

Sig Figs in Measurement

¬The certain digits and the estimated digit of a measurement are together called the significant digit of the measurement

Accuracy and Precision – tell about the reliability of a measurement