Measuring Substances

To measure chemicals requires different strategies depending on the state of matter.

• Solids: Weigh out the substance
• Liquids: Measure volume
• Solutions: Use the solution concentration to calculate moles

# Running Example

### Example:

1L Aqueous Solution has 571.6 gm of H2SO4 and has a density of 1.329 g cm-3. Determine the molarity, molality, mole fraction and mass percentage.

Density = Mass/Volume. Therefore Mass = Density * 1L = Density.
Therefore Mass = 1.329 Kg.

# Measuring Concentration

Concentration of X is written as [X] -> e.g. NH3 [2]
There are many different ways of expressing concentration

## Molarity/Molar Concentration

• Molarity = Moles per Litre
• Concentration = Amount of Solute/Volume of Solution (C = n/v)
• However this changes with temperature, as Volume increases/decreases

### Example

• H2SO4 has an atomic weight of 98.08 (g).
• 571.6/98.08 = 5.827
• 5.827/Litres of Solution * 1L = 5.827 Mol L -1

## Molality

• Molality = Moles per kilogram of solvent
• Concentration = Amount of Solute/Mass of Solvent

### Example

• 1.329 Kg - 571.6 g = 757.4 gm of H2O
• 5.827/0.7574 = 7.69 Mol Kg -1

## Molar Fraction

• Molar Fraction = Not a unit, just what fraction of moles is X
• Concentration = Amount of Solute/Total Moles

### Example

• Weight of 757.4/18.016 (mass of H2O) = 42.04 moles of water
• Mole Fraction = 5.827/(5.827+42.04) = 0.12

## Percentage Composition by Mass

• Percentage Composition = What percentage of the mass is X
• Concentration = (Mass of Solute/Total Mass) * 100

### Example

• 571.6 gm/1329 gm = 0.43
• 0.43 * 100 = 43%
page revision: 5, last edited: 14 Mar 2013 06:37