Precise information for measuring masses and quantities for chemistry practical.

QUANTITIES AND MASSES

Chemists have balances for determining mass in grams or kilograms. They have graduated glassware for measuring the volumes of liquids and gases. There is, however, no simple measuring instrument for determining chemical amounts directly. Instead chemists first measure masses or volumes and then calculate the chemical amount.

Masses of chemicals and amounts in moles.

The most direct way to find amount of substances to weigh it on balance.

Amounts of substances/mol =( Mass of substances/g)/ (molar mass / g ^ mol)

This relationship gives rearrange to:

Mss of substance/g = (amounts of substances/mol) *( molar mass / g ^mol)

NOTE:Include the units in relationship and use them to check that the units on both sides equal sign are consistent.

Quantities of gases.

If the temperature and pressure are fixed, then the volume of a gas depends only on the amounts of gas in nmoles. In other words the volumes of a gas is determined by the number of gas molecules present.

The law applies so long as the molecules of a gas are so far apart that their volume is insignificant compared to the volume of the gas and so long as the intermolecular forces can be ignored. For many common gases these criteria apply under normal laboratory conditions. Under conditions where the gas is close liquefying the simple rules do not apply.

Avogadro's law follows from the ideal gas conditions:

pV=nRT ( where,R is the gas constant )

If p and T are constant, the volume is proportional to n, the amounts of gas in moles.

The molar volumes of a gas is the volume of 1 mole of the gas. when n=1

p=(RT/V)

substituting in this relationship gives for the molar volumes of any gases which behaves like an ideal gas. Two sets of conditions are commonly used for comparing amounts of gases:

• For accurate work the quantities are calculated or standard temperature and pressure,s.t.p. The standard temperature for gases is 273 K and the standard pressure is 101.3 kPa(1 atmosphere). Under these conditions the molar volumes of gas is 22400 cm^3
• For approximate work, when making estimates under laboratory conditions it is often convenient to use the fact the molar volume of a gas at around 20 degree celsius and 1 atmosphere pressure is about 24000 cm^3.

Amount of gas/ mol= (volume of gas/cm)/(molar volume/cm mol)

NOTE:Avogadro's law states that the equal volumes of gases contain equal amount of gas molecules, in moles, under the same conditions of temperature and pressure.

Quantities in solution Chemists measure the concentration of solution in moles per litre:

concentration/mol/dm^3= (amounts of solute/ mol )/ (volume of solution/ dm^3)

This rearranges to give:

amount of solute/mol= (volume of substances/dm^3) *( concentration /mol /Dm^3)

NOTE: A litre is cubic decimetre, dm^3.