Chemistry 400 Ch 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 MeasurementSI Unit & AbbrevUS Unit & Abbrev MassKilogram, kgPound, lb LengthMeter, mFeet, ft TempKelvin, KFahrenheit, o F AmountMole, molsame CurrentAmpere, amp, Asame LuminosityCandela, cdsame TimeSecond, sec, ssame

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: GigaMega KiloDeci CentiMilli MicroNano Pico

Metric System PrefixAbbrevMeaningExample GigaG1x10 9 (1,000,000,000)1GW = 1x10 9 W MegaM1x10 6 (1,000,000)1Ms = 10 6 s Kilok1x10 3 (1,000)1km = 1000m Decid1x10 -1 (0.1)1dL = 0.1L Centic1x10 -2 (0.01)1cm = 0.01m Millim1x10 -3 (0.001)1mg = g Micro  1x10 -6 ( ) 1  A = A Nanon1x10 -9 ( )1ns = s Picop1x pg = g

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 °C.  How do you convert between K and °C?  They differ by ! K = °C OR °C = K  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 m 3.  Area is m 2.  Can you think of any?

Derived Units (Compound Units) Name, AbbrevSI UnitOther Common Units Area, Am2m2 Volume, Vm3m3 cm 3, mL, L, dL Density, dkg/m 3 g/cm 3, g/mL, g/L Speed, vm/skm/hr Pressure, PPa (kg/ms 2 )atm, torr, kPa Energy, E, H, GJ (kgm 2 /s 2 )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/cm 3 (1mL = 1cm 3 ) 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/cm 3, what is the mass of a 534cm 3 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?