CHEM 101 - TOPIC 1

Temperature conversion from Celsius to Kelvin

T(K) =T(^oC)+273.15

ΔT(K)=ΔT(°C)

Derived units

Combination of base units

Examples: 

Area: length x length  →  m × m = m²

Speed: length and time → m / s or ms^-1

Energy: mass, length and time

• kg ×(m²/s²) = J

1 m³ = 1000 L

1  L = 1000 cm³

1 cm³ = 1 mL

Density: Mass and Volume

• Two main physical properties of matter

• mass and volume are individual properties, but are related to each other for a substance!

Units:   g/mL (solutions),g/cm(solids), g/L(gases)

Extensive and Intensive Properties

Extensive property

• Depends on the amount of matter present.

• Examples: mass, volume, heat

Intensive property

• Does not depend on the amount of matter present.

• Examples: density, temperature

Exact vs. inexact numbers

Exact:

defined or easily counted

• no uncertainty (a dozen = 12)

• Infinite sig.figs.

• Does not limit sig. figs. in answer

Inexact:

• come from measurements or estimation

• has associated uncertainty

• may limit #sig. figs. in answer

What Is a Measurement?

• Quantitative observation

• Every measurement has a number and a unit

• Has uncertainty associated with measurement(not exact) (bottom of meniscus)

•– between 4.5 and 4.6 mL

•– next digit can be estimated

• maybe 4.56 (some may say 4.55 or 4.57)

– pointless estimating a third decimal point

Accuracy, Precision, Error

Accuracy

– how close measurement comes to “true value”.

1. Ex: Throwing Darts

• true value = bullseye

Precision

– how close a series of measurements are together.

1. Ex: Throwing Darts

Accuracy, precision, error darts example (poor accuracy, poor precision, etc..)

• Poor accuracy and good precision is when darts are together but on the side of the dart board

• poor accuracy and poor precision is when the darts are far apart from eachother and all not close to bullseye

• good accuracy and good precision is when all three darts are at bullseye all together

Percent error

• a measure of the difference between the accepted value and the experimental value.

Formula –

% error = (expected value - accepted value) / accepted value x 100%

• expected value is what you get

• accepted value is the lab value

SIGNIFICANT DIGITS

• Non-zero digits are always significant

• Zeros may or may not be significant

• three types of 0’s

  • leading is 0 in front of numbers even if before and after decimal point (NEVER SIGNIFICANT)

  • captive 0 is zeros in middle of numbers (ALWAYS SIGNIFICANT)

  • trailing 0 is the zeros behind the numbers (ALWAYS SIGNIFICANT)

  • ex: 0.008020

# without decimals:

Trailing zeros in a number with no decimal point may or may not be significant (placeholder?)

1300

• the zeros could be significant (measured) or they could be placeholders

Multiplication and Division with Significant Figures

• When multiplying or dividing measurements with significant figures, the result has the same number of significant figures as the measurement with the lowest number of significant figures

• (underline or subscript all non-significant digits)

Addition and Subtraction with Significant Figures

• When adding or subtracting measurements with sig figs, the result has the same number of decimal places as the measurement with the lowest number of decimal places

• More correctly: same uncertainty as the least certain number

Temperature in chemistry

Temperature in chemistry should always be in Kelvin when using a formula (i.e. PV = nRT)

Problem Solving: 4 steps

1. Sort. Identify given information and the find information you will need for your answer (conversion factors).

2. Strategize. Develop a conceptual plan. Do you have a formula? Use units to get to answer.

3. Solve. The easy part of the problem. Follow the plan to solve the problem, carrying out mathematical operations (significant figures and units are important).

4. Check. A step many beginners overlook. Always ask,

“Does this answer make sense?”  Errors easily creep into the solution (keys/transposing).

Unit conversions

1km / 1000m

1000mm / 1m

1h / 3600s

Problem Solving and Dimensional Analysis

• Arrange conversion factors so the starting unit cancels

  • arrange conversion factors so the starting unit is on the bottom of the first conversion factor

  • given unit x desired unit / given unit = desired unit

• May string conversion factors so you do not need to calculate intermediate results. Just make sure units cancel and final unit is one desired.

  • given unit x related unit / given unit x desired unit / related unit

MOST COMMON CONVERSIONS IN CHEMISTRY

g of A ↔ Mol A ↔ Mol B ↔ g of B

• the arrows are molar mass from periodic table

Mole of one substance to mole of another?

1. chemical formula (looking at how many moles in the formula)

2. chemical reaction (balanced) (use coefficients in balance reaction)