Chemistry 400Ch 1

Measurements

In the lab, we make measurements, that is we collect data with numbers and units.

The basic types of measurements with simple units are mass, length, time, amount, current, temperature, and luminosity.

Although there are many different ways of expressing these measurements, there is an approved system of units, the SI system. That way, any scientist can communicate with and understand the measurements that any other scientist makes.

Measurements

Measurement SI Unit & Abbrev US Unit & Abbrev

Mass Kilogram, kg Pound, lb

Length Meter, m Feet, ft

Temp Kelvin, K Fahrenheit, oF

Amount Mole, mol same

Current Ampere, amp, A same

Luminosity Candela, cd same

Time Second, sec, s same

Metric System

If we are using very small or very large numbers, we use the metric system of prefixes.

You need to memorize and use the following:•Giga •Mega•Kilo •Deci•Centi •Milli•Micro •Nano•Pico

Metric System

Prefix Abbrev Meaning Example

Giga G 1x109 (1,000,000,000) 1GW = 1x109W

Mega M 1x106 (1,000,000) 1Ms = 106s

Kilo k 1x103 (1,000) 1km = 1000m

Deci d 1x10-1 (0.1) 1dL = 0.1L

Centi c 1x10-2 (0.01) 1cm = 0.01m

Milli m 1x10-3 (0.001) 1mg = 10-3g

Micro 1x10-6 (0.000001) 1 A = 10-6A

Nano n 1x10-9 (0.000000001) 1ns = 10-9s

Pico p 1x10-12 1pg = 10-12g

Mass vs. Weight

Although you will hear many people (including chemists) use the terms mass and weight interchangeably, they are NOT the same!

Mass is independent of gravity. Your mass here is the same as your mass on Mt. Everest is the same as your mass on the moon.

Weight depends on gravity. Your weight is less on Mt. Everest and is much less on the moon!

We measure mass on lab balances. (but often just say weight)

Temperature Scales

Scientists all over the world use 2 temperature scales: Celsius (°C) and Kelvin (K)

The celsius scale is a relative scale. It’s numbers were set “relative” to the boiling point

(set at 100°C) and freezing point (set at 0°C) of water.

The Kelvin scale is an absolute scale. The lowest temperature possible in our Universe

(based on physics) is Absolute Zero, or 0K. This is the temperature at which all molecular

motion ceases.

Temperature Scales

Note that the Kelvin scale stops at 0. It can never be a negative number!

The Celsius scale can be negative. It stops at -273.15°C.

How do you convert between K and °C? They differ by 273.15!

K = °C + 273.15 OR °C = K - 273.15 Remember that the Kelvin temp is higher than the

Celsius temp.

Temperature Scales

In the US, we also use the Fahrenheit scale, °F. The Fahrenheit scale is also a relative scale.

It was based on our blood temperature as well as the boiling point of water.

The Fahrenheit scale may also be negative. How do you convert between °F and °C?

°F = (9/5)°C + 32 OR °C = (5/9)(°F - 32)

Derived Units (Compound Units)

Scientists also make measurements with derived or compound units.

These are units which contain more than 1 of the basic SI units (mass, length, time, etc.)

Volume is a derived unit as it is a compound unit of m3.

Area is m2. Can you think of any?

Derived Units (Compound Units)

Name, Abbrev SI Unit Other Common Units

Area, A m2

Volume, V m3 cm3, mL, L, dL

Density, d kg/m3 g/cm3, g/mL, g/L

Speed, v m/s km/hr

Pressure, P Pa (kg/m•s2) atm, torr, kPa

Energy, E, H, G J (kg•m2/s2) kJ, cal, kcal

Density

Density is a common compound unit and is an intensive physical property of a substance.

It is a measure of how close the particles of the substance are. Or how tightly the substance is packed.

It is mass/volume OR d = m/V. We commonly use g/mL or g/cm3 (1mL = 1cm3) for

solids or liquids. We usually use g/L for gases.

Density

Since we know that d = m/V, we can solve for d, m, or V.

If the density of a soil sample is 2.431g/cm3, what is the mass of a 534cm3 sample?

Density depends on the temperature, as you know if you think about water.

Usually, the density of a substance increases as it goes from the gas to liquid to solid phase (i.e. the temp decreases).

Is this true for water? What does ice do?