Chemistry 3.1 Using and Expressing Measurements,3.2 Units of Measurements,3.3 Solving Conversion Problems

Chemistry goal is F-B+

Measurement –

Finding how much of something there is using numbers and units.
Example: Measuring 25.0 mL of water in a graduated cylinder.

Scientific notation –

A way to write very big or small numbers easily using powers of 10.
Example: 6.02×10236.02 \times 10^{23}6.02×1023 atoms = 602,000,000,000,000,000,000,000 atoms.

Accuracy – 

How close your measurement is to the true or accepted value.
Example: You find the density of gold to be 19.3 g/mL (true = 19.32 g/mL) → accurate!

Precision –

Precision – How close repeated measurements are to each other.
Example: You measure 19.1, 19.1, and 19.2 g/mL → precise results.

Accepted value

The correct value based on reliable data.
Example: Accepted density of water = 1.00 g/mL.

Experimental value

The value you measured in your lab.
Example: You found water’s density = 0.98 g/mL.

Error

The difference between experimental and accepted values.
Example: 1.00 – 0.98 = 0.02 g/mL error.

Percent error

Shows how big the error is in percentage form. Formula (0.98-1.00)/ 1.00 × 100 =2% error 

Significant figures

The digits that show how exact a number is.
Example: 2.45 g has 3 significant figures.

International System of Units (SI)

The official measurement system scientists use worldwide.
Example: Mass = kilogram, Volume = liter, Temperature = Kelvin.

Meter (m) –

SI unit for length. Used to measure distance or size of lab items.
Example: A test tube is 0.15 m long.

Liter (L) –

SI unit for volume, used for liquids or gases.
Example: A flask holds 0.50 L of solution.

Kilogram (kg)

SI unit for mass (large objects).
Example: A tank of water weighs 2 kg.

Gram (g) 

Common chemistry unit for mass (smaller amounts).
Example: You measure 10.0 g of NaCl (salt).

Weight

The gravitational pull on an object’s mass.
Example: The same 10 g of salt weighs less on the Moon.

Energy

The ability to do work or cause change in matter.
Example: Heating water gives energy to its molecules.

Joule (J)

SI unit of energy.
Example: It takes 4.18 J to heat 1 g of water by 1°C.

Calorie (cal) 

 Another unit of heat energy.
Example: 1 cal = 4.18 J

Temperature –

A measure of how hot or cold something is (average kinetic energy of particles). Example: Water at 100°C has faster-moving molecules than at 25°C.

Celsius scale (°C)

Common temperature scale in chemistry;
water freezes at 0°C and boils at 100°C.

Kelvin scale (K) 

 SI temperature scale with no negative numbers.
Example: Water freezes at 273 K and boils at 373 K.

Absolute zero

The coldest possible temperature where all motion stops.
Example: 0 K = –273°C.

Density

How much mass is packed into a certain volume.
Formula: D = m ÷ V. Example: A metal cube with mass 10 g and volume 2 mL → density = 5 g/mL.

Conversion factor 

A ratio that shows two equal quantities in different units.
Example: 1 L = 1000 mL → (1000 mL / 1 L).

Dimensional analysis

A method to convert one unit to another using conversion factors.
Example: 2.5 L × (1000 mL / 1 L) = 2500mL.